Yglesias asks why advocates of performance pay for teachers are accused of teacher bashing since presumably as many teachers would benefit as would be hurt. I don't think this is any great mystery. The basic premise of such plans is that many schools are currently "failing" and that this is mainly the fault of the teachers at those schools. This seems like teacher bashing to me and unjustified at that since the premise is false.
Yglesias does identify a way in which teachers have brought this upon themselves. Teacher unions like to argue for higher wages on the basis that teachers are very important. This naturally suggests that perhaps greater efforts to identify and remove bad teachers are justified.
But while turning union propaganda against teachers in this way may have its attractions the fact remains that the evidence is that within the range of teacher quality currently found in US schools teachers aren't very important to outcomes. And, contrary to Yglesias, measuring individual teacher quality is very difficult as random variation will swamp any plausible differences in quality.
So the likely effect of any merit pay system is greater anxiety and uncertainly for teachers as some are randomly labeled bad with little if any benefit to students. It is no wonder teachers are largely opposed.