They're mad when you didn't tip at all. Then it was anger about not tipping enough. Now it's always tip cash, in a society where people aren't realistically going to the bank everytime they go to a restaurant. Next thing they'll complain about depending upon the size of the check, this is the lowest type we'll accept.
I really appreciate my F&B people, but they gotta stop with all this shyt. Take that shyt up with your boss and ask them to leave a "cash tip only" sign in the front or find a job somewhere that practices that.