It's actually nothing to do with idiocracy, the food labelling laws are very prescriptive. There would be a given list of food allergens and the legislation would clearly state that any food or food produce containing these allergens must contain the prescribed warnings whether it seems "obvious" or not.
If we were to go down the route of only putting warnings where we feel there is scope for ambiguity on the part the consumer we leave the door open for manufacturers to interpret the law as they see fit which would leave consumers potentially vulnerable. Also it would probably mean the drafting of legislation to be horrendously lengthy as there would need to be annex upon annex of products listed stating which product are required to carry such labelling and which are exempt.