I have to go back to my college books but if I recal it gets back to the 1930/40's when the CAA was running things and they used to put out united states postal contracts out for bid. Same thing today, most USPS contracts are done by FedEx or UPS. the majors do carry some but as a third party member so the rule does not apply. It makes no sense but we still operate under the RLA FROM 1936 which gives us no rights to strike.