Because the requirement to take cash is only for payment of debt. Debt means you've received something before paying for it, so all they have to do is not put you in that situation. For example: If they serve your food before you pay, then you are in debt, and you can pay that debt in cash regardless of their preferences. But if they withhold serving your food until you pay, then at no point are you in debt, and they can refuse to deal with you.