"I note that there are still no explanations how fingerprinting lawful American citizens will help protect us more than preserving our free and open society would."
Hey, maybe that's how you preserve your free and open society, by checking people's ID at the border and at airports to make sure they are who they say they are?
They knew who these people were. As others have pointed out on the previous thread, months ago, the fingerprints couldn't have told them anything about identification of these people--there's no database to compare to for citizenship. And the fact that they released these people after fingerprinting them just shows that they actually did know who they were--citizens.
Meanwhile, we allow non-citizens to stroll across our borders every day down south.