This came up in discussion recently so I thought I’d post it here.
Though not enforced very often (like marijuana by the Feds in Colo), if the patient has Medicare, providing pro-bono medical care falls under the Fraud & Abuse provisions, i.e., choosing not to charge one person while charging others.
The only time I recall this being enforced had to do with a physician doing a lot of other fraudulent things as well. It is also the genesis of the legal elimination of formal “professional courtesy” that physicians used to provide for each other.
I think the origination of this line of thinking came from an ACLU case with the basic theory that having providers pick and choose who they bill for what or at what level opened the door to widespread discrimination. This information was presented at a PSL Grand Rounds about 10-12 years ago.