The requirement is NOT that they have to treat people w/o insurance, since some people without insurance can still pay for their treatment. The requirement is that ERs must treat people whose lives are in danger, regardless of ability to pay.
That law has nothing to do with the individual mandate in the ACA. The law is based on compassion and humanity. The ACA doesn't change the public policy on compassion and humanity. The individual mandate is an administrative law for which there is a monetary penalty, and has nothing to do with receiving health care.