The receipt could have something to do with keeping the parking attendant accountable. I can't think of any specific situation where this could happen, but perhaps some attendants figured out how to charge somebody cash, pocket the money, and then wave them through as an AP holder. If everybody gets a receipt, though, it would be pretty obvious to someone that their receipt comes up "free, no charge" despite their having paid $20+ that something strange is going on.
I'm sure it doesn't work exactly that way, but my bet is it's meant to defend against something similar.