This is a strange framing. These laws are neither about what is taught in schools nor what books schools are required to stock, but rather restrictions on what books schools may chose to make available to the children. The government is not limiting the free speech of the authors, but these laws are the government limiting access to the authors' free speech, which is at least related to free speech, even if you don't buy that it is an restriction of free speech per se.

I do, however, think it is also worth noting that there is value in critically discussing the ideologies espoused by "The Bell Curve" and "Mein Kampf", since both ideologies persist and continue to have influence on American politics today.