There is a brewery in my town that does the same thing. They do accept cash and any "tips" (which is any amount over the total) goes to their charity of the month. On their cc entry system, its never shown to the customer so there is never that awkward tipping part you see on a lot of POS systems now. If the customer asks, they say that they don't take tips and if they'd like to leave one in cash, it will all go towards their charity that month.

By doing this, they appease the customer who wants to leave one and don't get in a back and forth with them over tips.