They're not just dumping bad debtors; they're dumping unprofitable debtors. During the height of the credit boom, AXP went nuts with low fixed-rate balance transfers for the life of the balance and jacked up customer credit lines like they were on crack.
Same thing. You don't pay your bills (ie bad debtors) then you are unprofitable. Their model was built originally on people paying off their balance completely every month, they did make the move to change that, they saw that it was unprofitable because of bad debtors, thus, they go back to what works. Good business move.