I would NEVER feed my child a PB&J in a shopping cart.
Of course, I pretty much don't feed my children anything in shopping carts. Those things are germ receptacles.
Now I'm rather annoyed I can't bring myself to do it, though. These people are insane.
Look, I'm sorry your child has a deadly allergy (which may have been made so by our INSANE reaction to such a minuscule portion of the population that has such deadly allergies), but it's YOUR job to keep them safe.
If incidental touch of remnants of peanuts means they are going to die, you probably shouldn't be bringing them out in public.
(Also, I believe that treatment exists to move the allergy from "YOU'RE GOING TO DIE" to "that gives me discomfort and pain." Look up some doctors that will work on "training the immune system" for peanut allergies.)