If there's nothing dangerous in the bag, it doesn't really matter whether it flies with the passenger or not... And every bag [I]is[/B] X-rayed (and many are hand-searched, as we all know from those friendly little TSA cards that pop up in our checked baggage).

It's the same reason it doesn't really mater whether a passenger gets on the wrong plane--once they've cleared security, they don't have anything dangerous with them.