>The GPL does not, and has never required anyone to "contribute back". Its whole purpose was always to "pay forward" by giving your users the same rights you got. If you're your only user, you don't have to share the code to anyone.
Oh come on, that's clearly against the spirit of the license. The central point that the FSF makes is that all software should be open (as in, the source code should be freely available). Paying forward the rights that you got (to view and modify the source code) means allowing others to view and modify the source code.
I disagree. Their point is that the user should always be able to share, modify and run the software they have, not that they should have access to every line of code written. Again, from the GNU project:
In general we do not believe it is wrong to develop a program and not release
it. There are occasions when a program is so useful that withholding it from
release is treating humanity badly. However, most programs are not that important,
so not releasing them is not particularly harmful. Thus, there is no conflict
between the development of private or custom software and the principles of the
free software movement.
Nearly all employment for programmers is in development of custom software;
therefore most programming jobs are, or could be, done in a way compatible
with the free software movement.
I doubt their modifications to the kernel (remember we're not talking about userspace stuff here) are "so useful that not releasing them is treating humanity badly".
Oh come on, that's clearly against the spirit of the license. The central point that the FSF makes is that all software should be open (as in, the source code should be freely available). Paying forward the rights that you got (to view and modify the source code) means allowing others to view and modify the source code.