Well, there's clearly no legal obligation. With the number of twists and turns in these programs and sudden, ephemeral bonus offers, we can't possibly be held responsible for auditing the airlines from a legal perspective.

Morally, it's obviously dubious. You cannot morally keep quiet about an unearned benefit. It's no different than accepting too much change at the grocery store (also legally permissible).

Having said that, I don't know how many of us can be rational about miles - a subject we're so fanatical about.