Rules range from the 1996 Health Insurance Portability and Accountability Act (HIPAA) which can be a major source of confusion on privacy and security issues, to the 1989 Stark Law which regulates doctor referrals, to the Corporate Practice of Medicine laws which prohibit businesses from employing physicians, and forbid doctors from splitting fees with non-doctors.
Typically, older workers would sign up for Medicare Part A hospital coverage at no cost but not opt to pay for Part B doctor coverage or Part D drug coverage while equivalent corporate coverage is in place.