No Guns/No Money
Why a generous tip? The tiny sign behind the bar was not what got you in trouble, it was your waitress getting her panties in a twist because she saw a scary gun. She didn't want the gun in her sight, she shouldn't be rewarded for running to the kitchen to tattle tail.
Not saying you should tip her zero, but certainly not generous. Compute the tip, round down, maybe let her know why.
The way I read it, the GUY noticed the weapon, not the waitress. If she was oblivious and didnt say anything, I dont see why she should be penalized.