The trouble is, without some overriding authority defining what it means to "have a plane ticket", what counts as "included"? Because anything that doesn't can then be considered an "add-on".

Carry-on luggage. Meal/snack and beverage service. A pillow and blanket. A seat that's not a middle seat. Even the ability to choose your seat at all.

Airlines that want to tighten the screws on their passengers can, in theory, start charging for all of those, and calling them "paid add-ons", even under a "no junk fees" law, if we don't clearly define what passengers should be able to expect to be included in their ticket.