Just require a tiny amount of rent, payable to the city, for correctly parked scooters. (This would be part of the scooter registration cost.) Then, if the scooter is left outside the approved sidewalk furniture zone, the company is issued a small fine (which is easy to enforce since the scooters must be registered and labeled with a barcode) that is designed to reflect the actual tiny cost of misplaced scooters (e.g., $5/day, not punitive) and incentivize the company to find a way to get their customers to leave scooters in the correct place at a reasonably high but not perfect rate.

Forcing each company to rent a million tiny places around the city from private parties is just highly inefficient. The transaction costs would be prohibitive.