I'm thinking it goes something like this. The TI calculators have something called "press-to-test", where a teacher can hit a combination of keys to disable functionality on the calculator. When in this mode, a LED flashes on the calculator so a teacher can tell at a glance that the student isn't "cheating".

Now imagine a homebrew OS where you hit the "press-to-test" keys, and the light blinks, but all the calculator's functionality is still there. If this became prevalent, then schools could outright ban the calculators on tests, so students would be less likely to buy them. Lost sales from this would dwarf any pickup from home hobbyists.