No one would complain about unionized teachers if they were doing a good job. All people see is vast sums of money going in and functionally retarded children coming out.
This could be because of bad teachers, bad parents, stupid/unteachable children, etc, but the unions' only solution is always "more money." Is it unreasonable to think, then, that the unions are acting in bad faith? The teachers are the only ones who get to elect the union leaders so they are responsible, directly or indirectly.
There's a huge shortage of qualified teachers because of the nonsense teachers ed requirements for certifying teachers. You've probably seen many articles on how ridiculous and shoddy these curricula are. These requirements exist at the behest of the unions to protect their members from competition.
A) (perception), unions are more concerned with benefits for their members than educating their kids. Uh... so? Who else is going to advocate for them? They have to eat too, and every small town in America trots out a bunch of reaganish bullcrap about how the school system should be able to do more with less but they all expect a raise in their private sector job if they're doing well. We drop our kids off at school for the day, drive to work on clean, safe streets, and then bitch about how the public sector employees should get by with no yearly raise, without even bothering to inform ourselves about our local budget.
B) (reality) Unions are oftentimes more concerned with the preservation of power, access and position for the top union officials than they are with either the benefits for their members or the quality of education, except for cases where the latter two feed into the first (fortunately, fairly often). This is how it actually works. Unions have their problems but it's not because the teachers are greedy -- teachers are already doing a difficult job for low pay, presumably because they care about education. Anyone who whines about unions or about teachers expecting a fair pay check is free to try it for a few years - last I checked there's a huge shortage of qualified math and science teachers.