Particularly with teachers who are committing actual crimes, though in fairness, I think that difficulty is overstated (point of note: I'm in the midwest, where the unions are not as strong, so that colours my perceptions a bit).
That said, though, I don't think we have any way of determing who is/isn't a good teacher if we haven't yet determined what good teaching looks like (in turn, we can't determine that until we figure out what we actually want out of school).
As a teacher, I've been on all sides of this; while working in a large public school, I've had lots of freedom and leeway (so long as I could justify it to my administrators, who were either down with it or completely and totally indifferent).
I've also worked in a small school where the "system" I was expected to carry out was complete and utter rubbish ("Students should not write at all until at least their junior year. They should spend the time prior to this diagramming sentences and memorizing the parts of speech, so that they understand what good sentences look like. They may only begin writing AFTER they have mastered identifying these patterns.")
When you couple that with an authoritarian administration, I was a "bad teacher" in the latter system, and had I not resigned I'd have likely been the first to go when budget cuts rolled around the summer after I left (as it was, my successor was let go instead).
I agree that incompentent teachers are a problem, but there's more to it than just that. Frankly, so many things have been screwy for so long (William Pinar, out of the U of British Columbia, places it at about 4 decades) that I'm amazed that anything gets done.