Re: I've missed something here
"I'm not sure a general purpose OS is suitable for this"
Maybe, but just because they started off with general purpose Linux doesn't mean thats what they'll end up with. From a security point of view any sensible designer is going to strip off all code that isn't needed, and the bit that's left ought to be there only because it is needed.
However, before Linux fans break out the champagne, I'd note that the US have yet to rationalise their many different air forces, armies and drone units (including at least six separate major air operations). The vast amount of overlap, duplication, infighting and waste, when combined with a budget squeeze that has barely started make this consolidation inevitable.
It doesn't follow that the USNAS will be the lead developer for pilotless choppers - could be the marines, could be the air force, could be the army, could be a new combined defence development unit. Until the consolidation has been completed there's no guarantees as to what hardware or software will prevail.