Re-registering a used Apple product

Why can't Apple simply "link" the serial number of an already registered Apple product instead of blocking it from being re-registered? Allow the re-registering of a used product and link only the serial number without divulging any identifying information or data. That way people purchasing used Apple products can keep track of their products under their Apple ID Account. It does not make sense to disallow re-registering of re-purchased products. I would like to be able to keep track of all my Apple purchases in one place even if they are not covered by warranty. It seems like a huge waste to only be able to use your Apple ID Account if you are under warranty. What about newly purchased applications and music? Why is it such a big deal? Seems like it would enable Apple to keep track of equipment better. I realize this is not their primary goal but if they want to be customer service oriented, it seems like the thing to do. There are millions of used Apple purchasers that could use the service.