I'm not sure about it being a legal requirement in all states (may be) but is generally done by the boards that oversee one's specialties (with some very vague and inconsequential requirements by the state licensing boards).

Unfortunately it seems mostly to be a money grab in many specialties, with passing rates in the high 90s for recertification exams, fees of over a thousand dollars, and often with multiple retries if one fails (as long as you pay the fee for each attempt).

So they are being taught by the pharmaceutical industry?