The final term of the zlib license state
- This notice may not be removed or altered from any source distribution.
However, the Wikipedia page lists
The way I understand the above quoted term, it means that the source code must retain its license when redistributed.
There are two possible ways I can see that this would not be considered copyleft:
- "from any source distribution" refers to verbatim copies, and thus a modified version, or a project created using the zlib-licesned software, would not be bound by this restriction.
- It is not considered copyleft because, while the zlib license would remain in effect, it does not have to be the only license in effect, so, for example, you could add a term that required those who distribute binaries of modified versions of the software to also distribute the source, while not directly conflicting with any of the three requirements given.
Is my interpretation of the license correct? Why would it then not be considered a copyleft license?