Bird clearly maintains the right to control the device because it unlocks the scooter to be used by customers and causes a light to blink so it can be found by "chargers" at the end of the day.

Regarding statutory violations, first of all Bird is probably not littering because the scooters are (arguably) not trash. Second, where the scooters are impounded by cities and other municipalities, it is for parking violations or other statutes against them. As a private citizen, you would have no more right to take their scooter than you would to take my car that was illegally parked.