Both tests permit the non-CAS version of the calculator, but only the SAT allows the CAS as well. Why?
They’re different tests administered by different organizations, so they can set their own rules. Besides that, I don’t really know why (perhaps ACT is more algebra-based?).
I’m not opposed to banning all calculators on either test, except for maybe a simple 4-function calculator (like the GRE does). Computer algebra systems on calculators are pretty powerful, and a lot of new SAT math questions can be solved only by knowing how to use the CAS functionality.
Not sure that is true for your model, my D’s Nspire has a Ti-84+ optional keypad too. It is allowed in both test with that keypad.