Does the Creative Commons CC BY-SA 4.0 license make it mandatory to share derived work?

Question

Let's say that I'm using art works protected by the Creative Commons CC BY-SA 4.0 license. I then modify the art works so that they fit my personal requirements and include them in a commercial software which I sell to my customers. So far, this all seems to be in line with what the license mandates.

But the license also states the following:

ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.

My mind comes up with two different ways to interpret that statement:

1. If I decide that I want to share the modified art works I have to do so under the CC BY-SA license.
2. The license forces me to share the modified art works.

Which of these two interpretations actually applies here?

If the first interpretation applies, would I be allowed to reject requests to hand over the modified art works?

If the first or second applies, would I be allowed to charge money for the work that I invested into creating the modified art works?

Answer 1

That summary is indeed ambiguous. CC BY-SA (and copyleft Open Source licenses in general) do not force you to share your work. Instead:

If you share your work, then you can only do so under the same (or compatible) license terms.

This is more clear in the full CC BY-SA 4.0 license terms. Section 3(b) describes the ShareAlike condition:

In addition to the conditions in Section 3(a), if You Share Adapted Material You produce, the following conditions also apply.

1. The Adapter’s License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-SA Compatible License.

   ...

where the “Adapter's License” is the license you use for the Adapted Material you create based on the CC BY-SA licensed original.

As a practical example: all StackExchange content is CC BY-SA licensed. I am allowed to take your question and create Adapter Material from it. I can keep my changes privately on my computer. No one can compel me to disclose them. However, if I share your answer with others, and if no copyright exception applies, then I'll have to do so under the terms of the same license.

You asked some further questions:

If the first interpretation applies, would I be allowed to reject requests to hand over the modified art works?

Yes. You decide whether to publish or otherwise share your work.

If the first or second applies, would I be allowed to charge money for the work that I invested into creating the modified art works?

Yes. The Creative Commons licenses do not prevent you from making money off the original or Adapted work, unless the license includes the NonCommercial clause (e.g. CC BY-NC).

However, everyone who buys the work from you also receives it under the CC BY-SA license. This gives the buyers the right to share the work themselves, gratis, and you can't restrict that. Because it is likely that others could legally get access to your work for free (once you've shared it), it is difficult to make money by selling copyleft-licensed works.