Maybe for security reasons. Let's say he wants to communicate with us and wants to sign the messages. For each signature he would need the private key kSat. With the above example he could prove it only once and then use the private key kabs for all signatures. If hacked, he could create a new kabsnew.
Or, far more simply, he could sign a message from 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa which says "I'm Satoshi, and here is my new PGP key", and then sign all future communication with said PGP key. No need whatsoever to mess around with adding and subtracting public keys, which is exactly the kind of nonsense known fraud CSW would do to try to distract from the fact that he cannot sign a message using the genesis address.