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.