The restaurant charges you, the plumbing company charges you, employees make whatever the business pays them.
You don’t walk out of a restaurant having just paid 3 dollars for your meal and drinks, and they didn’t even drive to your house,
You don’t have to tip the waitress anymore than you tip the plumber. Just pay the bill and leave. And you’re completely free to tip the plumber if you want. However, the traditional compensation system for waitresses has the tradition of tips built in.
Without tips, the waitress makes almost nothing, and what you’re really paying for with that tip is supposed to be the nature of the service she provided, whereas with a plumber, he can be a jerk or a nice guy and you don’t care nearly as much as whether he fixed your pipes.