Just a point of reference on those margins and why some places will be cash only. There's usually a $0.25-0.50 transaction fee for each purchase. On top of that, there's a percentage fee. Big businesses can qualify for a nice merchant account and will only be hit for 1-5%. If you're not big enough or well established enough, you'll have to go through a third party to take credit cards and that 1-5% can triple.
So for that hole in the wall food place to take credit cards, that five dollar lunch you want to charge, they might only get $3.75. And that doesn't take into account the machines and upkeep. Would you rather have a cash only $5 lunch, or that same lunch that costs $7 that you can charge?