Universal Orlando probably makes a ton off of people paying to ride on the things they already pay to ride on. I hate the idea of paying for it, but as a businessman, it's a good idea. Some people who visit Universal act like they have no choice but to buy the express passes, they'd no doubt be the same way at Disney.
My thinking is that the reason Disney has not charged for it is because, by limiting access to everyone (spreading out and timing it), it gets people to spread their vacation out over multiple days. Whereas, people could hit up pretty much everything in the resort within a day or two with an express pass. Sooner or later, the dollar signs are bound to show up in management's eyes, despite.
The resort fee is one way they're looking to charge for the service. I'd imagine, that's just one step toward it though. First the resort fee, then charging day guests while giving it to hotel guests by default (promoting it as a perk, of course).