More than any other job, teaching depends on a multitude of parties "doing the right thing" in order to be successful. Teachers are definitely one of those, but the best teacher in the world can't overcome parents who aren't involved with their children, a home environment and surroundings that don't value education, children themselves who may have been taught that teachers are "bad" and the public education system is "bad" so they want none of it and school administrations which are more interested in CYA than supporting their teachers. One, maybe two, of these can break down or be sub-par and a child still might get an education. But in many systems, you've got massive cascading breakdowns in all of them. Trying to then point out the faults in just one of them is then little more than blame shifting and finger pointing. Further, because of their intertwined nature, how can you fix one of them without fixing all of them? Any improvement in one area will slowly be ground down by the interference coming from the others.
Are there bad teachers? You bet there are. Maybe more than anyone would like to admit because having the desire to work with kids and education doesn't mean having the ability to navigate the current learning environment. But unless this evaluation takes into account the whole picture (kids, parents, administration, teachers and environment), it's just another bandwagon "Let's blame teachers!" torches-and-pitchforks battlecry. Even worse, its just bad journalism.
It also means the teachers who the evaluation call "good" are about to get all sorts of hell unleashed on them as parents read these things and then fight, sometimes quite viciously, to have Little Billy put into the top teacher's classroom or, upon seeing Little Sally is in a "bad" teacher's classroom, well, what's the point?