However, I also think they're way overpriced for what they are. I think a lot of the price is just 'because we can'.
It's a great deal for lazy teachers: not only do they not have to teach their students how to perform the calculation, they don't even have to teach the student how to use their calculator to do it. If a student wants to use a different model, they're on their own. They'll have to learn the TI BASIC anyways, because the textbook doesn't even try to do a decent job of describing an algorithm in any other notation.
In most cases, breaking TI's monopoly would be as difficult as, and possibly require, busting a teachers' union.
This adds big barriers to entry. You can't write your own libraries, and you can't use more powerful hardware to compensate either- TI's calculators cost them very little (the popular TI-83 uses a CPU designed in 1976), and will shove you right out of the market with their huge profit margin.