I'm not familiar with US law but I'm pretty sure that in the EU the warranty should be issued when they receive payment from you and the contract is established.
Which is basically how it works now...
They get payment, the item(s) ship, and warranty starts.
If you got your MacBook Pro, and the warranty did not start till you registered, and it was Dead on Arrival, what would happen?
Sure Apple would probably let you return it, but technically the warranty would never have started, ect. Then, you sent it back, and it hypothetically worked fine once Apple got it? Would this be sold as refurbished, or new, since it was technically never registered nor had a warranty?
It might interfere with the information Apple gets from returns, which then are flipped out as refurbished.