Acts like this are designed to have a psychological effect on the target. Since government can't outright just kill the guy any more, they will do everything they can to make his life miserable by doing things like this. Imagine being in Greenwald's shoes and knowing that your spouse is bearing the brunt of consequences for your actions. It would make many people rethink what they're doing.
My point is that holding him in an airport for few hours is piss poor attempt at intimidation. I've been subjected to worse at the hands of baggage handlers strike or a heavy storm.
