> One consideration could be that you can't directly control data ingress. Charging for ingress opens an avenue for attackers to attempt to bankrupt you by sending unrequested packets to your network. At the very least, this could cause a customer service headache. Perhaps AWS decided they'd rather not deal with that?

An equally likely reason is by making ingress free it helps balance out ingress and egress traffic flows which are usually desirable for settlement-free peering agreements.