For teachers, they argue that nonunion members get the same benefits as union members, so can be forced to pay union dues. Effectively, being forced into the union.

Marketing/coercion

My wife's part of the teacher's union only b/c they give you 1M in legal protection if you get sued.

I'm pretty sure this is why we as TAs join the union as a rule. Getting sued is a pretty scary thing. My buddy here said that is why he is part of the union.

Really? How often do teachers and TA's get sued? More than anyone else? Does that cover any lawsuit or just certain kinds?

Teachers: a lot more than you think. My wife has had threats to go to the schoolboard for retarded stuff (crazy cheerleading parents). And we know a couple that needed short-term legal counsel.

For TA's, I could imagine problems with patents, pharma, and medical.

I guess you could rig up the equivalent of malpractice insurance for certain fields to get around the union problem, but the current solution is the best available.

FDR era laws that give the unions a ton of power at the expense of both workers and owners.

Peer pressure. That's really about it.

I think it's common in sports.

You aren't quite forced to join the union. It's just a condition of taking the job. You can find another job instead. Sucks, but I think it's legal.