I was thinking of exams rather than day to day learning of basics, seeing tests as teaching rather than assessing. Point being you can't cheat at learning.
The calculator can't fake that you understand e.g. how to differentiate. The working is the bulk of available exam marks but the calculator only gives answers.
Is it unambiguous cheating if it doesn't help?
Knowing what the answer is supposed to be makes it much easier to reverse engineer the workings, and it lets you double check that your workings are correct. Also many of the more advanced graphing calculators (don't know the TI-89 specifically) have build in CAS systems and can do symbolic differentiation and as such help with showing your workings as well.
Plus the fact that these calculators let you store arbitrary data, so you can have your entire textbook stored in memory if you wanted.
I want to say something like "who learns advanced calculator functions solo but not the taught subject?" but of course shortcuts-by-rote spread like lice. I suppose that's the thrust of the problem with AI in schools