The school board banned it from the curriculum. That's a ban, even if it isn't a nationwide ban or a state-wide ban or any other kind of larger ban which no one has the authority to do anyway. Just because a kid can get a copy of Maus, doesn't mean the school board didn't ban it. Teachers are banned from teaching it. This is why people are freaking out.
But you are right, in a sense, because it's sold out on Amazon. Talk about the Streisand Effect!