From what I can tell, each scooter that is low on batteries is given a bounty (somewhere between $5 to $20) based on how hard it is to find. I assume that the "hardness" is computed based on how many chargers fail to find the scooter, but there may be something more sophisticated at work. To charge a scooter, a charger finds one, checks it out in the charger app, and then takes it home and charges it, and returns it to its "Bird's Nest" (clever) in the morning, at which point you get paid.
The scooters have GPS trackers to prevent theft.
So what these hoarders are doing is stealing the scooters -- rather than checking them out through the charger app, they're just grabbing them and tossing them in their van, and then keeping them in their home, where other chargers will fail to find them, thus increasing their bounty, until they "discover" the scooters, claim them through the app, charge them, and return them to their nest.
Have I understood this correctly? Given that they are removing the scooter without claiming it through the app (either as a rider or a charger) that means that they are stealing them, which means all Bird has to do is provide the police with the address of the hoarder and that's the end of it.