system
A chat on subreddit /r/NoStupidQuestions
human
Nothing happens, realistically. 

The server or bartender may make a mental note that you are a jerk. It's possible but rather unlikely that they could refuse to serve you the next time you come in, since in America you can largely refuse service for any reason (with some exceptions.)
gpt
I genuinely thought that because tipping was such a norm in the US, that if you didn’t tip it would be pointed out to you.