As someone commented on Schneier's site, US would have an incentive to keep Snowden protected because if any enemy of US would kill Snowden, then the information would become public, and I guess US wouldn't want that. Personally, I haven't invested a lot of thought into this, just wanted to point out an interesting angle. :)

They are not disincentivized from killing him. They merely are not being incentivized by a guarantee that the USA will have lots of sensitive documents released.

It should also be noted that an active dead man's switch opens the possibility that surveillance of Snowden will establish the mechanism of the switch, allowing the CIA to take over the switch and then do away with Snowden with impunity. By contrast the passive distributed key storage he likely has provides no active traffic that current surveillance can trace.

Perhaps the assumption is that the people to whom Snowden distributed the information wouldn't release the information (at least immediately) if Snowden were killed by an enemy of the USA. Of course, that's assuming a lot, including the fact that it would even be clear who killed him and why.