Allergen labelling is a huge blessing to anyone with serious allergies. Yes, labeling A Package of Allergens with 'Contains Allergen' might feel weird, but it's critical for non-obvious ingredients and contaminants.
So for consistency's sake it's better to just always list allergens, rather than have the regulations say "except when it's bizarre and nonsensical." Otherwise, someone has to decide where that boundary is.
I don’t think it’s really that hard to draw the line. You can still err heavily on the side of safety while not requiring a bag of nuts to warn about the nuts it contains. But our legal system doesn’t go for that sort of nuance.
The US law on allergen labeling does actually have this nuance, but sellers probably chose to err on the side of caution. See section 203 (a) of the Food Allergen Labeling and Consumer Protection Act of 2004:
> If it is not a raw agricultural commodity...
> ...except that the name of the food source is not required when ... the common or usual name of the ingredient uses the name of the food source from which the major food allergen is derived
Why not have a warning label? It's, what, perhaps 4 words extra?
How do you draw the line? Is the exception "if the item is exactly X and labeled exactly X then it doesn't need to contain an allergen notice for X"?
Or is it any broader? What specific knowledge should we assume that everyone knows?
If someone sees the name "milk", should they assume the product contains dairy? What about soy milk? What about peanut butter or coconut cream - do they contain dairy? Does Grape-Nuts contain grapes or nuts?
You might say that all of those should be obvious. But why make that assumption at all?
Isn't it easier to say that all products containing one or more of a given set of allergens must list those allergens? Because that's a really easy line to draw. It's clear to the producer, clear to the consumer, and clear to the legal system.