We don't know what they got. Perhaps some of them were paid to create the contributions. And, in any case, that's OK. The contributors knew or should have known the impact of the license. They could've picked a more restrictive/free license, depending on your point of view. I guess they can still revoke the license. They have not given up their copyrights and the license is arguably not irrevocable.

I'm sure their lawyers will be looking into it, you probably don't need to be concerned!