Advice invariably found in the literature handed out by purveyors of so-called "alternative" or "complementary" medicine, in which customers intending to buy quackery are advised to check with their GPs first to find what "mainstream medicine" has to say.
The venomous diatribes against real medicine, and science and rationality in general, behind the closed doors of the "alternative" movement should tell you all you need to know about the sincerity of the quack-my-ass clause. On the face of it, it sounds obvious, egalitiarian and big-hearted. However, the real intention of the advice is to ensure that if anyone dies or is incapacitated by taking the quack's advice or products (or by swearing off real medicine, which may not be advised on the packaging but is a stock in trade in the "alternative" industry), if the matter comes to court the quack's lawyer can claim that the product or the service was misused; obviously, they didn't check with their GP, how unfortunate, it's not our fault.
Besides, the quack knows perfectly well their client is unlikely to see their GP or specialist about whatever the problem is, or if they do they won't pay much attention to their advice. If they did, they wouldn't be coming to the quack in the first place.
Ah, here's the booklet; 123 symptoms this product may be able to cure, 256 further lists of types of people the product may be able to help, 25 more natural products from the same factory that might be able to balance your energies and so on, and, oh yes, the quack-my-ass clause.