When it comes to medicine, for the most part I really don't care whether or not people want to treat their illness with whatever they think will work. I obviously don't want people to die because they thought that homoeopathy could cure cancer, but for the most part I'm not concerned so much with every quack treatment someone thinks will work.
However I am concerned when those medical choices directly affect other people. For every person who tries to treat a cold or flu with vitamin C and zinc means that what's contagious is not being treated. In effect, many of our medical decisions have a direct medical impact on other people.
There's only so much we can do in terms of individual prevention, the communal nature of many illnesses mean that our responsibilities lie beyond just ourselves. Not just to ourselves, but to others too, we have an obligation to work towards finding out the best possible information and acting on it. If your health depends on me doing the right thing, are you really not going to be attributing some blame towards me if I acted against what was the best information for the sake of my beliefs?