The difference is that USO will give you that Express Pass for free if you stay at their Deluxe resorts which are far less expensive than Disneys Deluxe resorts, which only give you an extra 30 minutes of park in the a.m, and at MK only half the park is open for that 30 min.
But what if you stay at one of USO moderate or value resorts and have to spend the $120+ on Express Pass? It's still a deal because there value and moderate resorts are still less expensive then WDW moderate/value so the cost is well worth it and is comparable to rack rate at a WDW Deluxe moderate
The average person willing to spend the money that Disney wants to "get in on" appreciates a good deal, not paying $750 for a Deluxe resort (and that price includes AP discount) and getting 30 minutes in Tomorrowland at 7 am as a "perk".
But their are plenty of rubes out there too. I think Disney knows this and will gladly take their one-time earnings from that family who won't visit again for 10 years, if ever.