From what I learned, the coinbase field in the block reward transaction can carry any data (As long as smaller than 100b).
Why that actually? Was it easier to implement in the beginning? I would just have defined that the coinbase transaction may have no input. I know it is useful now to carry more information (see BIP34). I'm just not sure what was Satoshi's intention here, although he included himself the famous
The Times 03/Jan/2009 Chancellor on brink of second bailout for banks
Further it seems that this is missing in the transaction db and is unspendable, but I may confuse things here.