I understand that tips are supposed to be gratuitous and optional, but (where I live) waiters are paid 3.75/hr minimum. By not tipping, even when service is shitty, you are fucking another member working class out of a portion of their paycheck.

That being said they probably won't make under minimum. I don't know if this is the same everywhere, but restaurants in my area are required to pay their servers back up to minimum wage if they make under minimum throughout the week.

It seems that tipping is an American thing though. I would argue that the root of the problem is that servers aren't paid minimum wage plus tips, meaning that income is essential and not gratuitous.