[tuckermi]

I am unsure what distinction the linked page is trying to draw between payment for goods/services and debt. The first paragraph seems to unequivocally state that businesses do not need to accept cash (from a Federal law perspective anyway).

  There is no federal statute mandating that a private business, a person, or an 
  organization must accept currency or coins as payment for goods or services. 
  Private businesses are free to develop their own policies on whether to accept 
  cash unless there is a state law that says otherwise.

[ForkMeOnTinder]

I could be wrong, but my layman interpretation is that it depends whether you pay before or after receiving goods or services.

If you pay before (ex. grocery store), they can refuse to accept certain payment methods.

If you pay after (ex. restaurant), you've already eaten the food and now you owe a debt.