But nobody saying "license plate" is suggesting this - is there some reason why not?
Chalking seems 100% unreliable and very open to legal challenge.
I've seen smartphone based license scanners, Psion-based data entry terminals (they punch in each and every offenders license plate), but parking meters tend to have a paper receipt you place on the dashboard or stick to the window. But now they have a pay-using-an-app option ... They take your licence plate details always and check it against an online db, and if not then they look for a paper ticket.
Pretty much, if you park in a time limited street zone or a metered zone or a municipal car park ... You can be assured your car number plate is captured along with entry and exit times.
The data is likely sold on too
From a short search, it seems that license plate numbers aren't even unique across state borders in the US, for example.