You don't revoke passports. Once you arrested someone, the judge may decide to retain the travel documents to avoid that person fleeing justice. But the passport is not revoked, it is confiscated. And that is done once the person is arrested, not while the person is sitting somewhere in the world in a transit area.
Revoking a passport is quite extreme and I have never heard of such action. It is not the usual way to pursue international criminals. Thus it is a different treatment.
Passport Canada (US must have something similar) has a description of actions that may get your passport revoked. At this point I think he does fall in there.
Revoking Snowden's passport also violates this from what I can see as by removing his passport they're removing his right to travel and hence to leave Russia.
Or in other words the US has pretty much now completely thrown the de-facto document on basic levels of standards of human rights entirely out the window.
Owning a passport/travelling between countries is a privilege not a right. When someone is suspected of a crime and there is a good chance this person may seek to leave the country to evade prosecution, the passport will be revoked. Snowden is not a special snowflake to warrant a different treatment.