But in this particular case, in order to comply with the instructions telling you to throw the butter away, you need to know that this is the butter you bought that actually contains milk but isn't labeled so.
The very act of noticing that the butter doesn't have the right label tells you that it contains milk.
If it wasn't made from milk, it would actually be illegal (and factually incorrect) to label it as butter.
An analogy would be if you picked up water that wasn't labeled as containing hydrogen, in a hypothetical world where hydrogen must be labeled, and you concluding that this water must be made without hydrogen.
Anyways my point is that it just causes confusion in a system where, normally, you can simply trust that the labels are following the law.
That’s why they say to throw it out. 90% of people will ignore the recommendation. Some will dispose out of an abundance of caution, some will dispose because they had the thought I listed at the top of your comment.