Matt Cutts, who was/is a semipublic Google employee wrote on Quora today about his work at USDS.
It's impossible for someone to make the best decisions for American citizens while they're simultaneously employed by a giant international corporation which is evading billions of dollars in taxes, causing the rest of us to have to pick up all the slack.
Also USDS is a bit more "do this mission" style job: basically you're being dropped into a federal agency to try to turn their tech story around. 18F is there to promote longer-term change in the model of how the federal government does technology.