Earlier quoted context omitted.
It works because he maintains a high approval rating in his party.
Pardoning Snowden is not likely to be popular is his party, most Republicans aren't very big fans of Snowden. However, it is true that Trump is known for publicly "floating" a lot of ideas that he latter drops if they prove unpopular. But, if everybody cheers him for it it might actually make it more likely to happen.
Republicans have grown far more circumspect regarding their faith in intelligence services and Federal LEOs. Many see them as just players in and tools of the establishment political/media complex. At this point I don't think pardoning Snowden would hurt Trump with most Republicans. Certainly the 'Never Trumper' wing of Republicans would involuntarily soil themselves, but that's neither unusual nor significant.