system
A chat on subreddit /r/NoStupidQuestions
human
A server or proprietor could point it out, but it would result in a socially awkward situation. It's not generally done, because most people ultimately understand the tip is optional in the end. Theoretically you still want that customer to think well of your establishment and not tell their friends about the asshole bartender or whatever.
gpt
Oh so it’s like an unwritten rule. Tipping is technically optional but in practice it is never optional. It is expected but cannot be pointed out due to the technicality.