I believe that my daughter's teachers (and most other teachers/schools) really do want to help kids. But there's a lot of evidence indicating that many or most of them don't have the mathematical background required for making proper evaluations of the the flashy textbooks that get pushed by the big publishers. And yet that certainty comes in here: even in that blog post, the author emphasized that the books should be written by educators. She didn't really mention subject experts with graduate degrees in mathematics. This is almost as depressing.
It seems to me that they need both types working together on textbooks. You need someone who can say whether the material is being presented appropriately in terms of kids' developmental levels, but, obviously, you also need to make sure that the math is right and the theoretical background is strong.
I'd like to point out that while our kids are being served poorly by these books, it's even worse for struggling students. A really badly written question can transform even a good math student into one who looks bad on paper some of the time, unless/until they develop the "Hmm, what they really meant was..." skills HowlerKarma was talking about. Kids who are having more difficulty are likely to mess those up every time. That clarity is SO incredibly important.
