Perhaps people have always expected that a coke and peanuts are included in the price of a ticket, along with being able to bring a bag or two to accommodate a change of undies for on a short vacation. An airline charging someone to carry spare underwear is like a Jiffylube charging a person a $6 disposal fee for the oil they replaced, even though they advertise a $24.99 complete oil change, with fine print.
But on the other hand, folks don't normally expect that Fido is included in the boarding allowance. i.e. if you want an air filter or windshield wipers while getting an oil change, you expect to pay extra.