This requires a change to grant structure and cannot be done unilaterally by universities.
How many graduate students (outside of a few par time programs) make more than $25k a year? It doesn't make sense to consider parents income since graduate students are generally considered independent (even if they are being financially supported by their parents).
How do you stop everyone from qualifying for tuition waiver? Even rich parents will just make sure their kids don't have any assets if they plan on going to grad school.
>it should be taxed
We tax what we want to tax, and we offer deductions to encourage behavior that we think is beneficial. Scholarship income used for tuition isn't taxed. We tax capital gains differently from ordinary income. We offer mortgage interest deductions.
Should is subjective. There is no inherent reason to tax tuition waivers the same as employment income, apart from that's the way the IRS does it now (absent this specific deduction).
Currently, these students are paid a small salary to attend these programs ($20-$30k per year), and their tuition is waived. Under the new tax rules, the tuition waiver counts as income. So for example, a student at MIT who makes ~28k/yr is going to be taxed like they earn $80k/yr.