Earlier quoted context omitted.
Chelsea Manning stood trial. Edward Snowden did not.
Standing trial isn't a prerequisite for a pardon. Nixon never stood trial either.
Pre-emptively pardoning Snowden before he spends a day in prison sends the signal that the government isn't going to punish people for leaking, so go right ahead.
Commuting Manning's sentence after seven years still leaves a pretty harsh warning for future leakers (is leaking that information worth seven years of your life?) while showing a degree of mercy to the individual (I mean, not the degree of mercy that commuting after 2 or 3 or 6 years would be...).
Regardless of what you think Obama's secret inner beliefs and motivations are, I think it is reasonable to assume that as Commander in Chief and President of the United States he wants the people he entrusts with secrets to keep keeping them.