This is kind of bizarre. The folks over at the Nieman Lab have pointed out that when the NY Times launched its paywall, one of the things they said was that in the event of a "9/11-like" story, they can push a button and pull down the paywall to make sure everyone had access to the news.
And yet... the head honchos at the NY Times decided not to do so for the news of Osama Bin Laden's death.
The argument for why really makes very little sense.