why should USPS be treated any differently?
Because they are
government business. Their existence is legislated -- and they MUST serve 100% of US territories. UPS and Fedex are private, for-profit companies with investors and shareholders, and is free to serve whomever and where ever they wish.
If you remember DHL... there were LOTS of places in the US where they didn't run. They wouldn't deliver packages there at all -- no hand off, NOTHING. And UPS/Fedex do not hand off to USPS unless you pay for that service. (and you'll be told about it up-front.) [Newegg and Amazon use that service, even in serviced areas; I've had numerous USPS last-mile-deliveries to commercial locations in Raleigh, where their drivers have to go *anyway* for pickups and empty the drop box.]