I'm not sure what the city does, but the bike posts seem fairly clear of bikes, so I'm guessing they much remove them frequently. The univerisities all put notes on the seats for a week, then go and clip them off and donate the bikes to either the on-campus bike library or to the bike co-op.

I'm on the bike co-op board, and whenever we're running low on decent bikes to fix up and sell, all we have to do is call up campus security and we'll have another pile to pick up.