Identity

I have just returned from a shopping trip in a nearby town. At a branch of a large electrical retailer I bought a vacuum cleaner. Once the box had been scanned and I had refused the "extra" insurance, I was asked for my postcode. I said that I was not prepared to give it as I did not think it was necessary. The girl at the checkout said that it was something to do with the manufacturer's guarantee. I said the guarantee was in the box and I would have the till receipt as proof of purchase. She was clearly at a loss. I asked her if we could proceed by bypassing the address section shown on the screen in front of her. She said that she couldn't. Stalemate.
We then had a discussion about personal information and she agreed with me about living in this current climate of identity theft, fraud, junk mail and the general lack of privacy. However, it was quite clear that we were getting nowhere. A queue was forming behind me and I saw that the young lady at the checkout was in an awkward position. Reluctantly, therefore, I gave her my postcode and house number. She breathed a sigh of relief and went on to the next screen which was phone numbers, etc. She clearly though it wise not to ask, so went on to the final screen and I completed my transaction.
I really would like to know how this particular retailer can justify the requirement for these details. If I had bought the vacuum cleaner from my local electrical retailer I would not have had to give them. Also, were they really prepared to lose my business if I had stuck to my guns?
It seems that I have more and more arguments about this sort of thing whether it be in shops or on the telephone. There is this constant demand for me to give up my personal details. I rarely give in but I often cut off my nose to spite my face by leaving a shop empty-handed.