In reply to Jeffr0.

The current court ruling on the google vs oracle case relating to copyrights of the java API is that API is not copyrightable, therefor by defining an API you do not get to claim any right on a code that use it.

This was always kind of obvious as otherwise everything PHP will need to comply with the PHP license and everything on linux will have to be GPLv2. This was always thrown as a FUD against the use of GPL software "if you have even one small GPL component your product will have to be fully GPLed". I never heard of a case relating to this kind of thing and therefor I assume that even EFF do not believe that by just interfacing with GPL code you become GPL if after so many years of popular GPL software being used all over the place, no one was sued for such it.

The other issue that might "inflict" GPL on plugins and themes is the inclusion of of snippets from core or from theme distributed with core. This is trickier but for almost every plugin and theme those snippets will fall under fair use because they are small and without any creativity, or under "obvious" as there is only one recommended way to do things in wordpress therefor you can not claim creativity and therefor copyrights on its usage (think of a main loop, or widget class, there is really by design only one way to write them).

Even the foundation do not show any sign it fully believes in the GPL claim as it preferred a pitiful shaming campaign or accuse of trademark violation against people that do not comply with the GPL. Never they actually sued for copyright violation.

Legal splitting hairs sidenote: It is not obvious at all that the foundation can even release wordpress under GPL as it do not require the contributors to declare that they are the owners of the code that they are contributing. It is possible that someone by mistake contributed a code that he developed for a client and therefor the copyrights for it belong to the client and not to the developer.

This is why I always say that whenever a developer needs a license advice he should go to a lawyer and not take anything posted on the net as a gospel. Copyrights is a complex thing and the law can even be different enough to make a difference in different countries.