Some merchants demand the right to photocopy Impact Visa credit cards. GSA prohibits the copying of these cards.
The merchant's motive is to verify the identity of the card user, or to start the ball rolling for a little identity theft.
I have had occasion to get people fired for photocpying such a card. It is highly probable that Home Depot is seeking to avoid this particular problem.
Home Depot may have some sound reasoning for this action, but if I was one of their stock holders I'd be mad as hell for the inept way they are introducing this policy.