I was searching this a bit more online and someone suggested another "good" reason for such a sign: backing in hides the rear-mounted license plate. I'm sure th

I was searching this a bit more online and someone suggested another "good" reason for such a sign: backing in hides the rear-mounted license plate. I'm sure that's the reason in at least some places (such as airport parking), as it is common for police to run the plates of every car both to determine how long you've parked and also to check for outstanding tickets. Most states in the US do not require a front-mounted plate.