But in the real world, a terrorist doesn't just spawn at the other end of de_dust. They know others in these social circles long before they're radicalised.
The current crop of terrorists also happens to be a few landings short of a pilot's license, so to speak: the "Four Lions factor" (named for the movie) refers to the hypothesis that the aggressors' incompetence has been a major factor foiling their plans. How are they going to use a VPN if they frequently blow themselves up because they get confused by time zones?
Not that I consider this policy good in any possible way. It's one more example of that country softly proclaiming "all men are created equal / with unalienable rights", then turning around and making visitors feel like Somalian hostages. But if I were tasked with unearthing terrorist networks, I'd definitely get entirely to excited at the prospect of getting access to the Facebook graph.