See, this is the bit that does not compute. If you're on a very low minimum wage and reliant on tips to boost your income, then surely you do your best to offer a good service. That's how the model works, isn't it. To suggest that the customer must always tip, even if service is bad, renders the model ridiculous. And most people have worked as wait staff or behind a bar at some point, so most people understand what is under the control of the staff and what isn't.
Good service gets tips, bad service does not. That way the industry is staffed by people who can do the job and those that can't... well, they move on to different work.