Drinking untreated water also only has the _potential_ to cause disease. Playing Russian roulette with a loaded revolver only has the _potential_ to cause death.

Luckily, science can quantify those potentials and determine when the reward outweighs the risks.

If we, as a country, decided to let people who drank unpasteurized milk or untreated water to die without burdening the medical system, it would truly be their own choice. Once a society decides it will care for those people at a cost to the general public, it becomes necessary to protect the public from the burden of their ignorance.