The GPL exists because Stallman knew that corporations seldom do the right thing if not required so by law.
Plenty of companies simply avoid the gpl because it complicates things, and stick to more permissive licenses for their open source efforts.
Plenty more just do what the fuck they want regardless of what the gpl says.
Indeed, and this is why the same thing happens in the BSD world. There are many examples of large and small companies, competitors, working on the same parts of FreeBSD.
Enlightened companies contribute to Free and Open Source software because it's in their best interest, not directly because of the license. Conversely, there are countless examples of companies who don't think it's in their interest, and willfully violate the GPL.