At the same time, however, there are plenty of companies that use software released under a permissive license without contributing back -- they release none of

At the same time, however, there are plenty of companies that use software released under a permissive license without contributing back -- they release none of their improvements, they don't contribute financially and so on. GPL adepts hope that it will force these "malicious" companies to comply, while doing no harm to the ones that would contribute anyway. Or, in their own jargon, that the freedom (to reuse that software for any purpose, including commercial ones) which made this possible is preserved. The results have been... mixed, as far as I know. Permissive licenses do allow many commercial players to "abuse" the system -- and make no mistake about it, especially companies like Google or FB are entirely disinterested in charity. "For the community" is a convenient PR stunt, but real life has consistently shown that every large company has enough ulterior motives to make community service an accident, not an intent. On the other hand, GPL (rightfully) scares companies away. Even fair players shy away because of the complexity and difficulty of the license, and developers are sometimes put off by the amount of politics involved. These things discourage legitimate contributors.