Edward Snowden must be prosecuted for treason because he stole approximately 58,000 top secret documents of which around only 3,000 have been disclosed to date.
Multiple copies were made, shared among multiple parties and at a minimum were carried to Hong Kong, by others through the UK into Germany. Also, a copy exists in Brazil and likely one or more copies is accessible from the cloud via the Internet.
There is no practical way these documents have been secured and protected from those who may be considered unfriendly to the United States. The volume of information stolen obviously goes far beyond just the info necessary to pursue the public service benefit of protecting our privacy and extends into true national security interests.
For these reasons, prosecution is a must.