I believe that the idea is that they are taking something without paying, and that taking without paying is some variety of theft.
So, if it is theft, it really isn’t an issue of free markets. It’s a matter of criminal law.
As I’m sure you’re aware, hospitals can and often do, take people to collections if they don’t pay. But going to an emergency room for treatment without insurance coverage isn’t a criminal matter. Or it shouldn’t be, anyway.