You have your logic *** about face, spathi.
Knife carriers do not shop at JD because it is their "outlet of choice". They shop there because Mr Paul has given them vouchers to do so. They will (like many other people) shop anywhere if they are given free vouchers to do so.
Nobody knows why the well-meaning Mr Paul chose JD vouchers for his largesse. But one thing's for sure - if he had chosen Argos vouchers the knife wielders would have shopped there instead (or, more probably, sold the vouchers to someone who will, as they may also do with the JD vouchers). One other thing Mr Paul may well have overlooked is that there is nothing preventing the knifers surrendering a knife, accepting his voucher and promptly selecting another from their collection with which to protect themselves. And that's probably exactly what they do.