Scooters are definitively vehicles. What would governments do if people just started parking bikes and scooters in free full-size car parking spaces :)
Scooters aren't vehicles licensed for public roads. So it will probably still be littering, of public roads and parking spaces this time.
But you're supposed to ride them in the roads in most cities.
Yeah, but those statutes came after cars were popular. Early on, I bet people were parking all over the place and it was people complaining about it that lead to those rules being put in place.
Since the majority of places don't have statues for scooters, the default actually is littering, it's no different than dumping anything else on the sidewalk.