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.