It is not known whether this is a bug or intentional, but the reference client never adds the genesis transaction to its index. Thus, its outputs cannot be spent.
Changing this would imply giving Satoshi an instant switch to fork the network. I'm sure we respect him a lot, but not enough for that
It could be changed but made spendable only after some block in the distant future, such as block 500000, where the likelihood that all current clients have been abandoned and upgraded is overwhelming. Once that block is reached and is long passed, the code enforcing the restriction can be abandoned.
A reason I would suggest that it
should be changed is it represents an easily overlooked exception to the definition of a valid transaction. In a future where there are as many choices of bitcoin client as there are SMTP servers today, I would submit that this is a risk that will grow on its own if not taken care of in the present "era", because many bitcoin clients will be written in the future that treat this as a spendable transaction.
We must know if that private key even exists anymore to do such efforts. Otherwise it is like serving fresh meal every day in front of White House in a case if alien spaceship lands some day and they want to eat. There is no other consequences of this first 50BTC being unspendable, right?
But if Casascius is Satoshi as I suspect he might have a motive to make these 50BTC spendable sometimes in future.
Spending subsequent transactions to the address the genesis transaction was made to is possible because those transactions are normal transactions that every bitcoin client knows about.
Does not Bitcoin require all coins in inputs to be spent? Or I must read the white paper and specs 4-th time?