What purpose do book bans serve when those same books can be bought off Amazon?

Most importantly, who should exercise control over what others read? Saying parents need to decide is just another way to promote fascism. If the parents aren't exposed to literature when they are taught, how can they then decide what their children should read? Most parents that are opposed to some books, admit to never having read the very books they seem to ban. I remember struggling over Huck Finn, delving deeply into it. Have those than want it banned? There are so many "banned" books that are fundamental to education.