A bunch of you sound like you think restaurant workers don’t declare tips as income.
Having about 80% of my dates/girlfriends/ex-wifes come from the service industry, either waitress or bartender - I know they claim tips as income. I also know they don't claim ALL of them as income but if a tip is on the card - then it's definitely claimed 100%.
Tipping in cash - what happens after that is between them and the .gov and the less the .gov the better.
