There's not a line in this that makes any sense. Sure, maybe you're only TA'ing 20 hrs a week max, but you're also doing research work well beyond those 20 hrs regardless of whether your advisor has enough grant money to let you escape the TA time sink by putting you on the clock as an RA for work you were already going to do.
And it's a joke to pretend that you should owe tuition. That's nothing but a tax dodge for the university so they can pay you even less (pre-tax) for the same post-tax take-home.
Why should you be paid to attend your classes or to conduct your own research? That is like asking to be paid to go to the gym.
If your employer pays more that 5k of your tuition then you own tax on it so the US tax system clearly sees tuition as a benefit. You can call BS on the cost of tuition but that does not fundamentally change the fact that a waived charge is a benefit. The US tax system was a hairs breath away from making all tuition benefits even institutional grants at the undergraduate level taxable.
And it's a joke to pretend that you should owe tuition. That's nothing but a tax dodge for the university so they can pay you even less (pre-tax) for the same post-tax take-home.