> I really don't know what people objecting to this imagine typically happens to old, unwanted books.

In the UK at least, people usually take them to a second hand / charity shop, who sort through them and send the valuable ones to auction (typically early editions, 100+ years old) and then either sell them themselves (for recent books that are easy to get rid of) or sell them to specialised second-hand bookshops.

Most of the specialised second-hand bookshops rarely throw books away, usually if nobody buys them after a couple of years they end up in the extreme discount piles (20p, 50p etc) and probably only trashed if they still don't sell from there.