Refusing care during a medical emergency is unethical. Period. Although it is not always illegal. Usually private services that do 911 have a contract with the county they serve. They are paid a set amount annually and they can bill for whatever they can get. The annual fee paid by the county ensures emergency service to all its citizens. Refusal of a 911 call in this case is illegal, immoral and a breach of contract.
Private services that do not have a contract with the county they serve have a right to refuse service to anyone if there is a valid reason. Non payment for services rendered is a valid reason. It is up to the county to ensure EMS is available to its citizens. That's what taxes are for.
It could be argued here that EMS has a duty to act or that everyone deserves the same treatment regardless of ability to pay and so on and so on. All that is good and fine and very idealistic. It is also socialist. While I personally would help anyone that needed it, if I reasonably could, that reaction from me would be a direct result of my spirituality. As it stands, (for now anyway), we are capitalists here in America and the dollar will unfortunately override compassion sometimes. It's business survival. You can't help anyone if you can't pay your bills. The system needs to be improved, but you can't bash a business for protecting itself.