First, at this point it's reasonable to expect passengers in the US to know that their booking is subject to availability in some limited number of edge cases. It's not particularly common but it's also very public knowledge that it sometime does happen.
Second, both parties have a cost to rebook - passengers pay a rebooking fee, airlines pay a bump fee (in a voucher if the passenger is foolish enough to accept that). Additionally, the rebooking fee for a passenger is significantly lower as a % of ticket cost than it is for the airline.