This seems to me to be probably literally true but, I think, not true in the meaningful, operational sense. The waitstaff is free to quit, yes, in order probably to be out of a job for at least a few days, to face the hassle of a job search and possible increased scrutiny from future employers, and to experience the same low wages and mistreatment unless they luck into a totally different type of position. It's like an argument that gas-price gouging is OK because I'm free not to fill up my car.
You absolutely are not responsible for underpayment by management, but, if you eat at restaurants where managers underpay, you are no longer a neutral third-party; you are now a participant in that system. To extend your argument, if you don't want to be responsible for helping to make up the yawning gap in the workers' wages, you are free not to eat at restaurants.