Is Snowden really that valuable to the US? Prosecuting him isn't going to be a PR coup in the way killing Osama Bin Laden or prosecuting war criminals is (quite the opposite in fact; it can only be damaging to the US govt image inone way or another), he doesn't know anything the US doesn't already know, and the "don't give away our secrets unless you want to rot in Russia" message is well and truly sent.
It's not a PR coup, they want to discourage possible whistleblowers by telling them: "if you reveal big stuff about us, we'll hunt you anywhere you go and eventually we'll find you and you'll pay".
