The government are saying its to try and stop metal thefts, but the real reason is TAX. Working in construction, the company I work for end up with on average £1-£2k of scrap per month, which is all sold for cash. But next year all that scrap money is going to have to go through the books.
The real reason may well be for tax but if it cuts down on the scrap metal theft/resale market who gives a s**t. One tax law that will actually welcomed by most!