Unity silently changed the contract a little while back to remove the "we can't change this contract arbitrarily without you agreeing" clause. Which is to say, the terms that most of these devs would have agreed to when they started their current games _was_ one where Unity agreed not to force contract changes onto them.

Which is why it is not unreasonable for the devs to be both pissed off and feeling like they might have legal recourse as well.