Why is the road tax payment system so absurd?

You buy a vehicle which tax has recently expired on.... you are given a slip by the seller, but they accidentally give you the wrong tear-off - "transfer to trader" bit instead of the "new keeper bit" - you are standing in a long queue at the PO with money, MOT and insurance in hand, and get to the front, but they won't do it because you have the wrong bit of paper. It's ridiculous - I WANT to give you MONEY but you won't take it - now as i've driven the vehicle here i'm a criminal because i have no tax!! how exactly does this system deter criminal activity????