For a task as easily-remotable as software engineering, I find this pretty ridiculous, myself -- but a great many software companies still live in the old mentality of "if I'm paying you, I want a warm body in my office". It's not entirely unreasonable (and it'd be downright expected if the position were requiring a national security clearance or somesuch); but for something as low-risk as game development, I really can't see the harm in just letting someone from anywhere in the world work for you. Just have frequent status reports with them and make sure you keep apprised of what work they're doing and what progress they're making, and they should be just as productive as if they were in your office. If you're REALLY paranoid that they will just take your money and not do anything, then get them to set up a webcam for you of their office and share their screen with you so you can watch them work. That's no different than what companies already do to most workers; it's common for certain parts of the company to silently watch your screen and make sure you're on task. Yes, they have nothing better to do all day.
So, working under the assumption that Valve wants someone to physically be there: Getting one or more Russians a US passport and then getting them a place to live in Bellevue sounds like a daunting task. First, they have to be willing to move; second, they have to qualify for the passport; third, they have to either have enough money on-hand, or get paid large sums up-front from Valve so they can establish a foothold here. And I'm sure Valve's time schedule for doing this is pretty short, because they want to proceed with this project (whatever that project may be), and I'm sure they've got a docket of highly-qualified candidates a mile long.