One of our younger relatives, is a chef, and he has warned many of his friends about what you posted: "Not to mention that there's a good chance they will spit in your food if one is rude to them!"
Absolutely. I have no doubt they do it to many peopla.
I have known for yrs to be very careful when I complain at a restaurant (which is pretty rare, but the funny thing, the more expensive it is, the more likely there is something wrong with the food/service.)