But he was legitimately authorised : he just abused that permission. How was Morrisons supposed to stop that ?
As others said last time this was discussed, they were supposed to do that by making it bloody difficult for him to do so, or at least to do so without being detected. By making it physically impossible to attach storage devices, for example. By making him pass through a metal detector, maybe - I have seen that in non-spooky environments. By logging what he was doing. And so on.
Do stores still have kitchen knives on open, non-secure display? I haven't looked recently, but I'd be surprised.