Lots of doctors who are attendings get paid for what they bill. But this change is for doctors who are 1st year residents/ interns. They haven't graduated to being an attending yet. Residents do not have a medical license yet and must operate under the supervision of an attending. In theory, they are getting paid a salary to learn / sharpen their skills in their chosen specialty (ie family medicine, internal medicine, etc.). But in reality, they are more like low wage labor as 1 attending can supervise many residents.