Easy to see that Snowden did U.S. citizens a service of exposing where normal checks and balances had failed in the collection of data on Americans. Sure he broke the law to do so, but the benefit is so obvious that a pardon doesn't seem like it should be that politically toxic at all.

Also easy to see that giving huge quantities of unvetted secret information to foreign journalists does with near 100% probability put that information in the hands of the Chinese, Russians, or other bad actors (relative to the U.S.). A journalist is a trivial target, as Snowden would absolutely understand. If it can be shown that there is information that impacts national security negatively, then for that subset of the stolen data he should face prison or even death penalty.