It is true that this "dumbing down" is market driven, Art, but it is also true that the market are state and local boards of education.

Textbooks have always presented things in a bland and overly general way. They tend to be blandly written, as well--good students generally don't get the depth that they need, and all the rest are turned off to the subject because the texts are so boring. For this reason, many teachers heavily supplement the official texts with other materials--which is much easier than it used to be, owing to the internet and destop publishing(but more expensive, because it consumes a lot of paper)--and there are more than a few teachers who never use the textbooks at all, in effect, publishing their own, and handing them out a few pages at a time.