"May contain peanuts" is not a label that would appear on peanut butter. It is a label on many products from M&Ms candy to canned cashews. It is on those products because of the horrible food allergies that many people have to peanuts. In many cases, people have had to be hospitalized just for being at a baseball game where others in rows ahead of them were cracking open roasted peanuts. This shows you just how minute a bit of the allergen may be while still having devastating effects. Because the factories where those products are produced also process products with peanuts, the manufacturer knows that small amounts of peanut many enter other products, and provides the label as a warning to the consumer.