I doubt satoshi owns the key of this genesis bitcoin address due to the fact that the 50 BTC is unspendable. So many ppl just send dust amount to this genesis address to destroy their bitcoin.
You're assuming a lot. The genesis block was the only block where in some sense Satoshi was guaranteed the coins, as by construction he was competing with no one. He may have viewed this as unethical, and therefore deliberately left out the code to add the genesis coinbase to the UTXO set - I find it hard to believe, though possible, that he wouldn't have realized that it wasn't in the UTXO set. I suspect it was a deliberate choice.
Once he had released the code and anyone could download it, he might have ethically viewed all subsequent blocks as fair game (though no one other than Hal has stepped forward to claim a block earlier than #78) and so mined for all he could.