For emergency treatments, insurance companies are required by the ACA to pay the out of network provider, but charge you as if you are using an in network provider.

It's possible for the out of network provider to then bill separately from your insurance if they charge more than what your insurance pays out of network providers, but there's no way that's what happened here because $1500 after the out of network fee from the insurance company is far too high for an ambulance trip.