Air marshals are a service the FAA provides to its public air passenger. They do a lot of work protecting the airspace, and often work alongside civilian air traffic controllers.
Basically, air marshals are a bit of a joke in the airline industry. They’re not really a service to the public, but to the airlines. It’s just the way it is. Air marshals are mostly civilians who actually fly for the airlines and in many cases are paid for by a commercial employer, such as a gas station.
Air marshals are employed by commercial airlines to work on their flights, and to protect them in any way they can. They are primarily used in airports, but its not uncommon to find them in the air when its a flight heading towards a destination. They’re supposed to be there to help the passengers in their flight, but in reality they are often there to just get a few extra dollars into the pockets of the commercial airline.