Where precisely did I go wrong here? We know it's butter, but a consumer might not, therefore a "contains whatever" label is not totally unjustified. Once you accept that as a rule, you have to enforce it. You don't just enforce it where it's ambiguous, you enforce it on all brands.

I could believe that "Signature Butter" could be a substance that wasn't butter. It would be dumb, but I wouldn't give you odds it didn't happen if someone claimed it did.

So putting milk in the ingredient list at least confirms that yes, this is butter, not "butter."