Note: this is a real example, but I don't care nearly enough to fight it; I ask purely out of curiosity.
I was at the dentist and I had a few cavities filled in. My understanding from what was explained to me is that 2 of them were filled with white, and 2 with silver; insurance covers 80% of silver, but 0% of white. The dentist never mentioned that there were options, and never mentioned that the options cost different amounts of money.
For that matter, I never explicitly agreed to pay for anything. What makes a medical fee enforceable?
Edit: I could understand situations where the patient is incapable of communicating/deciding, but nevertheless needs urgent treatment, and how in a situation like that the patient would get the treatment and subsequently be responsible for paying.