aaaah but they get to sell the ticket again AND get $70 from you.
Except it is BA that gets the $70, not QF. QF loses the revenue for the sector from BA (so there is a net dollar loss for QF) and doesn't really get to sell that seat again (it was already "sold" by the oversales algorithm). BA does get to keep the cost of the sector as well as the change fee, so nice for BA ... one of the bennies of running BAEC and setting the rules.