Agreed, it's insane to me that in an era of Google Colab (et al) schools still require students to shell out >$100 for one of these. I'm sure there is some backroom arrangement with schools of some kind.
Neither teachers nor school districts have the time or resources to audit every new tool someone wants to use, or to help students figure out how to use their preferred tool to do something - find something that works and just use that
I had a cheap Casio fx calculator. It got me all the way through my exams in school and university. I had Mathematica at home.
While I can see that being very good on a TI-84 would help you complete exams faster and get better marks, is that a skill that we want students to learn? Being good on a fancy calculator is essentially useless in real life. In real life people use computers not fancy calculators.
IMO it's better to either allow only basic calculators, or to allow real mathematics software.
Where to draw the line depends on the course. In general tools that "give the answer" for something where thinking provides insight are bad in education - for instance, a CAS which will simply compute derivatives isn't beneficial when taking Calculus. Things that eliminate grunt work not useful to that intuition - like computing the same formula 40 times to draw a graph by hand - are beneficial.