Even though funded graduate students are relatively cheaper than professors, they're unlikely to fire tenured or tenure-track profs unless it's absolutely necessary. Also, there are going to be people who accept offers from Berkeley unfunded because it's their dream school/they have outside funding/are independently wealthy and can afford it. Lastly, state school budgets are often set by the state government rather than the university itself, and they may have limited control over how much funding they are allowed to offer/what each department gets.
