It is rude to refuse to tip.
Sally found a hair in her salad which she ordered at a restaurant.
Sally wants to show how disgusted she is.
Sally gets her meal comped but proceeds to leave a tip for the server because it wasn't her fault.
Sally refuses to pay for her meal and tips nothing because of the incident.
Sally's server is happy to get a tip despite the bad customer experience.
Sally's server doesn't make enough money during her shift because of the kitchen staff's mistake.