At least here in California, just about all the teachers that I've met who are active in the union grumble about how reluctant administrators are to fire incompetent teachers. It seems like principals, though, are more often willing to backhandedly try to get rid of someone for political reasons than they are to openly challenge a teacher's competence. The only people that in the current system have the authority to judge whether a teacher is good or not are the school administrators, and there's little to no oversight to ensure that they don't play favorites (not too different from a lot of other jobs, I don't think). Of course, unlike many other jobs, teachers can still be highly effective even if they act completely against their boss's ideology.
Unions aren't interested in making it any more difficult to fire teachers. But you can't really blame them for being supportive of certain aspects of the status quo when they know that if the requirements for termination were more lax, a lot of those pesky pro-union teachers would be the first against the wall, regardless of how good they are at their jobs.