What a scary situation.
It brings to mind the fly-by-night private services who can be defined as questionable, at best, when it comes to their patient care, billing practices, and every other aspect of their operation. I think the biggest difference, however, may be that it sounds like the black ambulances make absolutely no effort to appear legit, where the services I was referring to will jump through just enough hoops to skate by for a while without being detected by officials.