Teacher's Unions and Catholic Schools

I know, not what I usually write about, but I find this interesting. Normally, the teacher's union is a pariah. They are no good, greedy, blood suckers. That is, right up until they want to unionize the teachers at our local Catholic School. Now they are heroes. How can this be? Don't misunderstand me, I have the utmost respect for teachers. I think they have a very difficult job and they take a lot of crap that most people couldn't take. I am just curious where the bashers are. Not one letter to the editor have I seen where the author brings up the strikes, summers off, etc, etc, ad nauseum. Anybody have any answers?