Not outrageous. It is totally consistent. They don't allow resale of govt. school or homeschool teachers answer books because they don't want the students getting their hands on them.
It makes "homework" too easy by looking up the answer.
Of course, a group of homeschool subversives could always take the texts used by govt. schools and publish "answer sheets" and put them on the web in order to bring down the public school system.
Not that I am advocating that.
So does the internet!!
Beyond that - it is the teacher's / parent's job to ensure that thier children are learning instead of cheating - not a Bool Store's, an internet auction site, a reseller, the government's or anybody else!