Fair enough, they can both be shit. I just don't see how it's my responsibility as a customer, to fix the problem that the resto is underpaying its staff.

The way I usually do my part to help workers who are not treated well, is to boycott the employer. If tipping is seen as a solution to that problem, I'd call that enabling. Tipping is the only reason restos can get away with grossly underpaying their staff. I don't want to support that.