If they don't make enough in tips to reach minimum wage, the employer still needs to close that gap. It's not as exploitative as we're led to believe, or at least no more so than working a minimum wage job is normally.
Federal minimum wage is 7.25 after taxes this is about 250 per week wherein most people in this situation are paid every 2 weeks. That is to say that one cannot expect to have more than 2 paychecks per calendar month for a grand total of $1000. If you didn't need food, clothes, medicine, or anything else this is about enough for rent.
What this means is that the employee/employer relationship not to mention the individuals ability to live indoors and have food is actually predicated on the ability to achieve greater than minimum wage. Tipping out even if they can do so isn't enough to keep anyone afloat. Not tipping in a state that has a tipped minimum wage is not that much different from dining and dashing its just not illegal because its never been illegal to screw someone if they are substantially poorer than you.