score:4
Answering my own question:
As stated in a comment by @DJClayworth, the US does not provide emergency care free of charge. Charges do vary between hospitals, and US healthcare is the most expensive in the world.
Visitors contemplating extending their stay in the US because of worries about the extent of the epidemic in their country of residence should check that they have adequate insurance for the entire length of stay, starting from the date they arrived in the US.