Actually, there is a restaurant where I go sometimes that when I pay cash instead of with a credit card, the owner gets so elated that rounds down the amount to pay in some 3-7%. The countertip I guess.
Yes thats normally the main drive, cash is usually just annoyance and additional risk to business.
I wouldnt be too harsh judging that business though, quite a few restaurants are barely cutting it so this may help them stay afloat. Its this or generally higher prices in restaurants.