When I used to work for United Express, the number one rule of conduct in our Flight Operations Manual was, No chewing gum. (really), but also in those rules was a statement that the pilot was to have NO contact with the public. "Management" thought we were going to start holding out our hat for tips. The also posted signs in the cabin of our aircraft, "It is out pleasure to serve you. Tipping is prohibited."
I always thought is was very obnoxious to tell the customer, "tipping is prohibited."
Stupid managers.