Multipe copies of "hot" books won't be kept forever. They'll supply the demand while it's there and then be discarded when people stop checking them out so fast.

My mother's friend does book sales for the library and a lot of their discards go to schools, nursing homes, etc., or are sold to raise money for newer books. They don't put them in the garbage (unless they're falling apart, of course).

"Weeding" is standard practice in librarianship and it's not a bad thing. We don't just "throw out" books randomly--we pull out ones that are worn out, whose information has been superseded by something more accurate and up-to-date, ones that aren't being checked out, etc., and try to replace them with something that better serves our patrons. It's not perfect and we can't make everybody happy, but we try to make it more a "tailoring" process than just elimination.

My library doesn't stock a lot of what I'd like to read, either, but they have a lot of people to serve and they're supposed to be accessible to everybody (including the handicapped, blind, deaf, non-English speakers, etc.). That's a lot of ground to cover on a chronic lack of funds.