Earlier quoted context omitted.
The way I see it, if you contribute to a project for example by sending a patch to it by email or pull request then you agree to be bound by the license. Even if there is no copyright assignment. Otherwise you would not have the right to modify the software to write your patch in the first place. This just seems like common sense to me.
The problem is if the maintainer wants to change the license (Like GPL2 -> GPL3). That's only possible if the contributors have signed over copyright, or the maintainer can track down every person who ever sent a patch, and get them to agree.
On the other hand, as happened here, it makes people less likely to want to contribute to the project, because they probably feel like they are being taken advantage of or losing some right.
A counter argument to that, anyone is always free to fork the project, again, what happened here, but this has the obvious undesirable effect of splitting development effort.