I'm not suggesting that anything nefarious happened. I do not think Snowden is an FSB operative. I think he grabbed a giant tranche of documents, few of which he actually read, and handed them off to a small set of reporters he believed to be trustworthy.
But the USG has an interest (a) in confirming that and (b) containing the damage.
By the estimation of literally everyone involved, Snowden took documents whose publication isn't in the public interest. Snowden and his informal team of assistants have charged themselves with protecting intelligence secrets. It's not crazy to think that the USG might want to relieve Snowden of that role.