Yea that might be it. But that doesn't strike me as unusual either. There are many situations where a criminal charged with a lot less has to surrender documents as a condition of being allowed out on bail while they await trial, if they're considered a flight risk. In Snowden's case, he knew he was going to face a lengthy criminal process and would be detained, so he (perhaps correctly) chose to leave before he made his revelations and before his movement was restricted.