Have a look at PostgreSQL, SQLite, LLVM, Clang ... all these are projects that get a lot of support from proprietary software vendors. I'm pretty sure that the companies contribute to these projects because of the liberal license, not in spite of it.

I think a big fallacy with regard to the GPL is that companies don't contribute to open source because the license forces them to. Companies contribute to open source because it makes more sense to collaborate on some projects rather than each work on their own.

Licenses like GPL etc. are great for ensuring their user's freedom; but they don't really incentivise companies to contribute to them at all. Instead, they prevent some companies from using them. So counter-intuitively project with liberal licenses end up with much more contributions than projects with strong licenses...