Make no mistake, Snowden is not "free" to leave Moscow.
Russia has him and while they were happy to let him spill shit that didn't involve them... I'm pretty sure that not all the secrets he has to spill would reflect well on Russia.

That's the only logical reason they would exchange asylum for his silence.

As for "free to leave" yeah he is, alone without any protection... so he's stuck there unless he wants to risk getting on a plane by himself, which he won't because he is naturally being shadowed by US spooks... probably sitting in the airport staring right at him the whole time.

He is fucked, no matter what he does now.

His only options are to either flee and get picked up by the US or remain a "free" guest of the FSB.

He's fucked.