In many, if not most parts of the world, once the agent tags your checked baggage and it disappears on the belt into the bowels of the terminal, it's using infrastructure belonging to, and operated by, the airport rather than the airline.
You can bet that service isn't provided to the airlines for free and the cost will be recovered by the airport, whether on a per-item or other agreed basis. I suspect that's a not insignificant proportion of the true cost to an airline of carrying passengers' hold baggage.