Board certification is completely voluntary, even for specialists like cardiologists and nephrologists. They can also still be issued malpractice insurance (although the premiums may be higher).
Your point is simply wrong. Especially in underserved rural areas you'll often see primary care doctors delivering cardiology and nephrology treatments even though they lack board certification in those specialties. There are no certified specialists available so they make do.
Nah, the point was just wrong. In many cases these primary care doctors are doing more than just maintaining a treatment plan prescribed by a specialist. Apparently you don't understand how the practice of medicine is legally defined in US states.