1.) Overwork their students, stretch them thin doing everything.
2.) Have more graduate students.
They would always pick #2. Professors are notoriously short-handed. They are typically not the ones who are outright abusing the power. It's usually the administration tying the professor's hands in these matters.
As a graduate student, your boss is usually the administration more than the professor - especially w.r.t. things a union would care about (workplace safety, wages, benefits, overwork...). Professors do not like to overwork their students - it's just they don't care if you are overworked.
