Blimey! roll on Saturday :)

2 seconds after I snapped this innocent shot in Tesco, I was approached by a very burly but polite security guard who informed me that I was not permitted to take photos in the store without the express permission of the store manager. I toyed with the idea of explaining the concept of blip and needs must and all that but I didn't think there was much reasoning with the guy. What are these shops always so afraid of? What harm would the odd snap do?