That's what is mind boggling to me. This idea that a nation would revoke a passport for someone to leave to a radicalized state... I mean, yes, you don't want ISIS gaining more supporters, but now you have an ISIS supporter/radical you are forcibly keeping within your own borders, as a free person.
There was an article in The Atlantic where a reporter interviewed several ISIS supporters who had their passports revoked after attempting to travel to the middle east.
Here's an article from February about France revoking passports of suspected ISIS recruits.
http://www.newsweek.com/france-confiscates-passports-six-suspected-isis-recruits-308929