The student groups signed the letter as legal entities. But not every single member of those groups necessarily agreed to it.
Members of at least some of those group were not even notified about the letter.
So you can only be part of groups you 100% agree with? I wouldn't care enough to 'post my disagreement', especially when I was a student, I had other stuff to care about.
The groups I belong to do many things I wouldn't do. Some of them I'm sure I wouldn't do. As for others, I'm not sure as I don't have enough data and time to verify them. So you imply I should leave them all? In this case nobody could belong to any group because sooner or later any group can do at least one thing you wouldn't do.
if they drink bud light while you prefer ipa, i guess you can get over it. if there are more serious moral dilemmas, then yea. you probably should evaluate compatibility.