That becomes a tough one, doesn't it? I'd say that the use of it without tipping incurs a moral burden, as you're giving the employer money and not making it right with the person who (as is true of anyone in a tipped job) depends on you upholding your part of the social contract.
You aren't saving the world by tipping--but you're not making it a worse place, and that's not nothing. Similarly, by not using a service that does its employees dirty without making it right, you are pressuring them--if, admittedly, mildly--towards decent behavior. That assumes that you make it very clear why you're doing it, and you totally should, because communication is part of ethical consumerism. (For similar reasons, when Amazon, Uber, etc. recruiters reach out to me, I underline the way they treat their workforce as a major reason as to my disinterest. It's a data point, and only one, but it's what I can do.)