A lot of things, actually. Milk is so clearly and consistently marked as an allergen that I'll often as a vegan just check the allergens if I don't have any reason to suspect the use of meat products, meat byproducts, honey, or non-allergenic dairy ingredients.

I would probably still do a double-take and check the ingredients here, but with the movement to plant-based alternatives, you never know if someone who treats this the same way I do as basically a gold standard (because that's what it's supposed to be) will simply take it at face value. It's also plausible that someone without strong English literacy but with such an allergy would rely solely on the basic allergen label rather than trying to parse more complicated English words.

The reason it has to be strictly enforced like this too is that if you justify this as "well everyone knows ~~it's Butters~~ butter, so it doesn't really need a label", then it's not as trustworthy and therefore efficient to those who need it, and it risks drawing a line where not everyone is on the same page.