> Snowden was never convicted of anything, how can he be pardoned?

You don't need to be convicted, or even charged, to be pardoned; a pardon doesn't even have to be specific as to the crime(s) it covers, it can just cover any crimes you may or may not have committed during some time specified time period before the pardon was issued (as was the case with Ford's pardon of Nixon.)

It is true that the usual clemency petition process operates after conviction, but that's a discretionary administrative process, it does not reflect the scope of the Constitutional pardon power.

Yes, you're missing something. Presidents can pardon before conviction, like when Ford pardoned Nixon.