He/She/They should, but having 4% of the total amount of a currency in the hand of one completely unknown person / organization is quite a threat to its stability. Also I would see a reward of one billion dollar as pretty excessive. And that is only the worth at the current market value.
Excessive based on what metric? Inventing the worlds first successful peer-to-peer crypto-currency? Zuck @ Facebook, and other startups (Square etc.) made their founders far richer. Do you realize how hard it is to build trust in a currency even if you are a government?
Strong parallels here to the case of equity in a new business. It wasn't worth $1B when they did the work, and the likelihood that it would ever be worth a significant amount was low enough that the reward would have to be high to be a useful incentive.
In addition, everyone who bought in over the years implicitly accepted this deal. I'm sure if the inventor had claimed 50% of the total currency pool, there would have been less interest.
