Overbooking is to accommodate those with fully refundable tickets, though it is those who buy cheap non-refundable seats who get bumped. "Non-refundable" tickets can usually be changed for a (fairly high) fee, so they are not locked in stone. Finally, based on their algorithms, the cheapest seat classes are likely sold at "filler" rates that do NOT cover costs and profits. An extreme equivalent is the $1 promotional seats sold by Megabus.