It's simple. The teacher/school would have to get permission from parents. Many parents would refuse to give permission. This would force the teacher/school to provide alternative material. That becomes a pain when part of the class is reading one thing and part is reading a different thing. Its just easier to avoid the situation all together by either "sanitizing" the book or using a different book.

By the way, thanks for the link. I just ordered a classroom version of the book for my kids to read (yes, I would be one of those parents who wouldn't give permission for their kids to read the original version.)