It sounds like most people are missing the point of what FastPass is all about and why Disney created it. Disney didn't create it as a special treat for their guests although they cleverly present it to you that way. If you read about queuing theory and Process Cycle Time, you'd see it's actually a very sophisticated tool to optimize the wait time, and here's the cool part, for all riders. By utilizing Fast Pass they divert people from the StandBy line which lowers the wait time for StandBy and then constantly adjust how many FastPasses they give out throughout the day and length of return time to optimize the overall throughput of the line which translates to a lower avg wait time for all riders. They're constantly tweaking that with this idea in mind. They have a vested interest in this since the shorter your wait time, the less time you're spending in lines which gives you more time to travel between rides which means more opportunities to spend money on food and stuff. It also means you'll be happier and willing to spend more time in the park which again means more opportunity to stay there for all of your meals, shop, etc.
All this said, FastPass already makes them a lot of money without having to charge you a dime for it. If they were to ever move to a pay model, they'd want to be careful that it doesn't undo the benefit they already get. Universal's model is very different in that the Express Pass is a lot more exclusive (not nearly as many people have it) and the result is that it really only benefits the Express users and not the StandBy people who actually end up having longer wait times. This probably drives up hotel revenue and the direct revenue from selling the Express passes but they might be missing some potential revenue from the StandBy people spending a lot of time in line. I can't speak to Universal's model too much but trust me that Disney has a whole lot of IT and data analysis paying attention to how their ride queues function. Also, Universal's model gives a pass for the whole park for the whole day while Disney's model allows them to tweak things on a per-ride basis. That extra control is important to them.