I have done this a couple of times. Once, at quik trip on a particularly hot day, I stopped in to have a cold drink. I actually hooked up as soon as I got there, went inside, and as I was posting for my drink, I said, "I hope you don't mind, but I plugged my electric motorcycle in to one of your outlets." The guy looked at me like I was crazy and said sure. I made a couple calls while I finished my drink, talked to the usual few curious onlookers, unplugged, and left. I could have made it home without plugging in, but figured I'd give it a try.
If they don't let you plug in, don't patronize the business. Seriously, we're looking at about 10 cents worth of electricity. F@ck em if they won't give you that much if you're buying something there.