As a recovering food service business owner I know this dilemma all to well.
Merchant agreements with credit card processors have pretty strict requirements in almost all cases. It's a violation of the agreement to charge more for credit purchases, discount for cash purchases (everyone older than 34 remembers when gas stations did this), or require a minimum purchase amount for credit. If you see one of these practices it has probably NOT been approved unless you're dealing with a government entity. That doesn't mean it doesn't happen on a regular basis and the only real penalty to the vendor is possible loss of privileges with the card companies.
As for the potential tax issues I can only offer my own experience. I had a mix of about 75% cash sales vs. 25% credit sales. I paid "sales and use" taxes on every cent that ran through the register or card machine, there were never any profits to tax so I skated by on that one... unfortunately! During frequent conversations with other small business owners they remarked about the nature of my cash business and the inherent tax "benefits" stating that cash in their businesses never saw the bank - this audacity often surprised me. However, I'm not naive enough to believe everyone is as straight-laced as me or jaded enough to believe that everyone is on the take. As we move in in the direction of a "cashless" society these types of decisions may be made for us in the future.