It is decided by both the state and the counties which companies can have licensed ambulances. Usually the state controls the criteria and standards a company must have to obtain a license. However, counties have boards that determine which companies may receive a certificate of need(which ultimately determines if an ambulance company can be in business in a county.)
If you would like to obtain a list of all of the licensed ambulances in a particular state, it is easy to do. Under the Freedom of Information Act(FOIA), a list is readily available. Simply call the state, usually the department of health, and find out from whom you can get the list of licensed ambulances. Tell them it is a FOIA request.
Whether a patient is being transported by air or ground, licensed ambulances are almost always used. If the patient is travelling by air, licensed ambulances can provide transportation for a patient to the airport on the sending end and from the airport on the receiving end of the flight.
There is a great difference in the amount that licensed ambulance companies charge for transporting a patient. Companies that transport patients by air sometimes have contracts with the ground transportation units around the country to try to save the patients money.