Why is CC BY-SA discouraged for code?

Question

Everybody knows that you shouldn't use a CC BY-SA license on your code. Even the Creative Commons folk recommend against it. What I don't know or understand is why. I can't seem to find any information explaining this.

I contribute as both an asker and answerer on Code Review Stack Exchange. Since SE asks us to license our contributions under CC BY-SA, I'd like to know the implications this has on code I post there. Most of my code is released under GPL3 or MIT otherwise. I know by posting it on SE I'm dual licensing, but what exactly am I agreeing to?

1. Why is CC BY-SA discouraged for code?
2. What implications does dual licensing have for these (sometimes quite large) sections of code have from my projects?

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Stack Exchange proposed an update to their terms of service in regards to code licensing in late 2015/early 2016 but that change is "delayed indefinitely"

Answer 1

CC's own FAQ addresses the reasons, which I find satisfactory, so I'm just going to reproduce it here and expand on the key points:

Unlike software-specific licenses, CC licenses do not contain specific terms about the distribution of source code, which is often important to ensuring the free reuse and modifiability of software. Many software licenses also address patent rights, which are important to software but may not be applicable to other copyrightable works. Additionally, our licenses are currently not compatible with the major software licenses, so it would be difficult to integrate CC-licensed work with other free software. Existing software licenses were designed specifically for use with software and offer a similar set of rights to the Creative Commons licenses.

Our licenses are currently not compatible with the GPL, though the CC0 Public Domain Dedication is GPL-compatible and acceptable for software. For details, see the relevant CC0 FAQ entry. We are looking into compatibility of BY-SA with GPL in the future; see the license compatibility page for more information.)

(Bold is mine)

That is, CC-BY-SA

• Doesn't mention source code
• Doesn't mention patents
• Isn't compatible with major software licenses¹

Why are these good reasons for not using CC-BY-SA?

It doesn't protect access to source code. The "-SA" part simply means, "I'll share this piece of work, and you can use it as long as you then share yours under the same terms". But there is no mention of alternate forms; this only covers the final, publicly-available form. So someone is perfectly within their rights to take CC-BY-SA code, produce a derivative software, and only share the binary under CC-BY-SA. By contrast, GPL explicitly mentions source code, and requiring the distributing source code, when you convey alternate forms such as binary form.

It doesn't protect against patent disputes. This may or may not be important, as evidenced by many software licenses also not mentioning patents, but for some use cases it is very important.

It isn't compatible with major software licenses. The FAQ specifically mentions GPL incompatibility, that is, it's impossible to combine and distribute CC-BY-SA and GPL licensed code in the same piece of software. Since the body of GPL-licensed and GPL-compatible-licensed software is much bigger than that for CC-BY-SA, releasing using CC-BY-SA greatly limits the usefulness of the code.

When choosing a license, you should define what you're trying to achieve with the license. CC-BY-SA provides much weaker protection than copyleft software licenses, since it doesn't guarantee access to source code, but because it is so incompatible with other licenses, it is more restrictive in practice. It's very likely that there are much more appropriate licenses than CC-BY-SA, for whatever you're trying to achieve. For example, if access to source is not an issue, try a permissive license. If you want to ensure that your code remains freely available, try a copyleft license.

¹: @kyll has mentioned that CC BY-SA version 4.0 has a one-way compatibility with GPLv3. This is specific to BY-SA content integrated in a GPLv3 project. Using CC BY-SA on still suffers from the incompatibility problem. From the FAQ:

Version 4.0 of CC's Attribution-ShareAlike (BY-SA) license is one-way compatible with the GNU General Public License version 3.0 (GPLv3). This compatibility mechanism is designed for situations in which content is integrated into software code in a way that makes it difficult or impossible to distinguish the two. There are special considerations required before using this compatibility mechanism. Read more about it here.

Answer 2

TL;DR:

1. CC-BY-SA is a technically perfect open source license.
2. CC-BY-SA's use is discouraged because of the "license proliferation" problem.

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Going into more detail on the first point, first we need to know what it means to be an "open source" license. OSI has a great definition of what it means to be open: http://opensource.org/osd

Lets go through each point:

1. Free Redistribution – The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution

This is the most important requirement for a license to be "open". CC-BY-SA does not restrict selling or giving away the software and therefore it passes.

Other creative commons licenses (such as CC-BY-NC and CC-BY-NC-SA) fail here and are not open source.

2. Source Code – The program must include source code, and must allow distribution in source code as well as compiled form.

CC-BY-SA allows distribution of the source code or the binary.

3. Derived Works – The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.

CC-BY-SA passes here too, you are allowed to modify the work and can distribute under the same terms.

4. Integrity of The Author's Source Code

This is an extension of point 3, clarifying some areas where an open source license can block distribution. CC-BY-SA passes.

5. No Discrimination Against Persons or Groups

6. No Discrimination Against Fields of Endeavor

CC-BY-SA is fine for these points.

7. Distribution of License – The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.

CC-BY-SA is fine again, you can redistribute without permission.

8. License Must Not Be Specific to a Product

CC-BY-SA is definitely not specific to any product - that's the whole point.

9. License Must Not Restrict Other Software – The license must not place restrictions on other software that is distributed along with the licensed software.

Just an extension/clarification of point 1. CC-BY-SA passes here.

10. License Must Be Technology-Neutral – No provision of the license may be predicated on any individual technology or style of interface.

Again, CC-BY-SA passes.

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CC-BY-SA fits the definition of open source perfectly and is therefore suited to open source code.

There is, however, one reason to avoid using the license: The issue of "license prolifiration", which OSI has explained nicely: http://opensource.org/proliferation and there's a wikipedia page going into more detail: https://en.wikipedia.org/wiki/License_proliferation

Basically, it's bad for the community to have a bunch of licenses that may or may not be compatible with other licenses. Far better if everybody settles on a small number of licenses where the community clearly knows which licenses are/aren't compatible with other licenses.

This is especially true for "restrictive" licenses like CC-BY-SA, since those are the ones most likely to have compatibility issues. Proliferation is less of an issue with "permissive" licenses (like CC-0) so the community generally is happy for those licenses to be used (although OSI and the FSF do not actively encourage their use).

This is why CC-BY-SA is discouraged. But you can use it if you want, and still consider your project open source.