Some countries also provide air traffic control services. Part of the fee goes towards these services. There’s no standard fee. Countries use different metrics to determine the cost. Canada’s fee takes into account airplane weight and distance traveled. The United States’ fee only considers the distance.
Airlines sometimes take longer flight paths to avoid high fees. But avoiding airspace is harder than it seems, because airspace can be larger than the country itself. U.S. airspace extends all the way to the Philippines. But flying over the ocean costs less than flying over land. Even though it costs less to fly over water, some airlines still have to pay the US— even if it’s just going from Australia to Japan.
WHY DO AIRLINES PAY?
Airlines pay because, without the payment, they are not allowed to enter the airspace.That means, an authorised, and unplanned access might be treated as an invasion by the country and they can respond with their air defence system according to their threat assessment, possibly in a hostile manner. No airline would want to have such record.