[pjmlp]

No they cannot unless they want to contaminate their development, as far as I recall the licenses aren't compatible.

It is true for any commercial application with source code available since there are computers.

If the user freedoms were of any value as you mention, WebKit and Bink would use copyleft licenses.

[jefftk]

Just because you can't literally copy the code from one place to another doesn't mean you can't learn from it. I'm not a lawyer, but my understanding is it's fine to learn what series of system calls they're making and look at their optimizations.

[pjmlp]

Reading the code means analog copy paste.