I happen to agree that this is a bit strange, and I think it is basically "government says do it, so you do it even if it doesn't make a whole lot of sense." But here are some additional examples that may shed some light on things:
On the one hand, Oreo cookies are commonly listed with an OU-D certification. This normally indicates "contains dairy". But the OU has publicly stated that, at least for the time being, as long as there is no "May contain milk" or "Contains milk" or any milk-based ingredients listed, these can be considered dairy-free from a Kosher perspective such that they can be eaten following a meat meal. Trust me, for Orthodox Oreo-loving Jews this was big news when it came out several years ago. As I understand it, the OU asked the manufacturer to put an OU-D on the package instead of OU (indicating no dairy or meat by default) or OU Pareve (clearly stating no meat or dairy) because this allows the manufacturer to change the ingredients (as long as they are Kosher) and manufacturing processes (e.g., baking in the same equipment used for products containing dairy) by changing the labels as required by government regulations (e.g., adding "Contains milk" or "May contains milk" as appropriate) without requiring a recertification by the OU to determine the labeling. In other words, OU-D covers all the likely scenarios from a Kosher perspective.
And on the other hand, several years ago I saw a Passover product which stated "May contain wheat, milk and nuts" and next to it a line "This has no Kosher concern." (or similar, I can't remember the exact phrase). That was important because the item was certified as Pareve (so no milk) and Kosher for Passover (so no wheat, or rather any wheat had to be special for Passover which gets to be a very big deal, and in this case no wheat intentionally included).
So there is one situation where the allergen statement is used to clarify the Kosher statement (is it really dairy or not) and in the other the allergen statement is essentially minimized (because Kosher rules are rather strict, especially for Passover) but the manufacturer felt the need to still include it based on their understanding of FDA regulations.
Margarine is basically vegetable oil, flavoring, coloring and other chemicals made as a butter substitute. People who keep Kosher look for margarine that is 100% dairy-free. Many margarines (and similar products) contain dairy. Many do not. It may not be obvious from the ingredients. While Kosher symbols work for those who keep Kosher, the allergen statement or lack thereof can be vital for those who don't keep Kosher (or on a product that is not Kosher certified). As an example, I can't believe it's not Butter may not have butter but it does have milk ingredients based on the allergen statement (and the OU-D) even though there is nothing obviously dairy in the ingredients.
This is the exact reverse of the peanut butter question. Many "non-dairy" creamers are actually made partly from dairy products! Really. Blindly following the name would be a problem for someone with a serious allergy problem. Looking at Coffee-Mate the label says: non-dairy but the ingredients include SODIUM CASEINATE (A MILK DERIVATIVE) and the allergen statement says CONTAINS: A MILK DERIVATIVE.
So what is in a name? If non-dairy creamer can include dairy, and "it's not butter" include dairy, who is to say that peanut butter must have peanuts?
Some comments on the original question got me hunting on the internet for what each manufacturer says as far as allergens. All are from manufacturer web pages. Except as noted, all are Creamy, the only type of peanut butter that matters.
- Jif "Contains peanuts"
- Peter Pan: "Contains: Peanuts"
- Smuckers: "Contains peanuts"
- Trader Joe's: "CONTAINS PEANUT"
- Smart Balance (not sure if this one counts, it is Chunky not Creamy): "This product contains peanuts."
- Skippy seems to be the lone holdout - it has "Ingredients & Allergens" which just lists the ingredients, the first one being Roasted Peanuts.
However, I just checked some of my actual real Jif Creamy Peanut Butter - and I can't find the allergen list anywhere! But the ingredients have PEANUTS in bold - I don't know if that is for emphasis or if it somehow satisfies an FDA requirement.
And just for fun, I pulled out a container of Lactaid Milk and sure enough, after the ingredients is CONTAINS MILK.