Liability avoidance is largely an American thing. It has to do with our medical costs and lack of universal health care. Also lawyers getting a cut of recovery out of contingency agreements. This was illegal under the common law and still is illegal in other places.But to address Jay's more existential question, WHY?, all of these disclaimers are to avoid liability.
For the other countries, I suspect the warnings are partly meant as a joke about Americans and partly to actually warn. If peanuts give you anaphylactic shock, you'd certainly appreciate a peanut warning. In my old country, you'd just die and that's just the way it is.