I believe Stanford University gets something like 40% of its annual budget from tuition (other top universities are similar). That's not huge, but it's big enough to make it totally infeasible to simply stop charging tuition.
So, yeah, the interest could pay for all the tuition, except X of it is restricted to the law school, Y to the med school, Z to arts and sciences but not in any way that it can contribute to tuition, etc.
Ah, and tuition money is particularly valued, because it's unrestricted....