The way I see it, this is an open source shim layer. I don't see any copyright issue whatsoever. Whoever chooses to use the shim is using ffmpeg anyway, so they are already exposed to the license in exactly the same way. This just makes it easier to use ffmpeg 'the right way' on various devices, if I understand correctly.

If somehow MS can't make and open source clean compatibility layers for their own software, then that's a big problem.

ffmpeg is also distributed under LGPL, which would be necessary to link this library against it, otherwise the whole product would be GPL anyway.

> If APIs are copywritable

Is that distinct from being copyrightable?

I'm sure that if it becomes a big enough problem, the problem will be buried in lawsuits once Microsoft or IBM or similar stub their toes on it.