I cant believe some people's arrogance. You were asked to produce a receipt for goods from the shop which you already had in bags, big deal. Why shouldnt you be asked?? They have to deal with shop lifting every day which in turn hits us all in the pocket when they have to put the prices up to compensate. Fair play to the shop assistant I say, they were doing their job, and I think preventing shop lifting is part of their 'policy', jeeeeez! No harm done.
This has nothing to do with 'freedom', you were in their shop, on private property, they were simply asking for proof you purchased the items. Not unlike walking out of a cinema to go to the loo and walking back in, and a staff member asking to see your ticket.
Anyhoo, well done to the OP with the publications!