Also, when you know something contains peanuts, you just stick a label "CONTAINS PEANUTS" on the packaging. You don't get the grocery store to ask everyone for

Also, when you know something contains peanuts, you just stick a label "CONTAINS PEANUTS" on the packaging. You don't get the grocery store to ask everyone for a peanut allergy certificate before selling them a jar of PB, and also keep a log of who bought PB from them. And we already have something similar for porn sites, in the form of the "Are you over 18?" landing page banners.