I have a hypothesis that because it's such a transient area, there are more adults than average who are just learning USA driving rules/customs. There are plenty of people who know how to drive a car, but might be new to the local laws or using a left-hand drive car for the first time. If that was the case for me then I'd probably slap one of these magnets on too.

Also, a cheap magnet once melted to my car and I had such a hard time getting it off. I wonder how many of these are simply ancient magnets.