The people leaving Europe for Jihad have a far greater chance of dying on the battlefield than their brethren already there. Taking away their passport and deporting them to one of those countries IS pretty much a death sentence.
. . . as it is for their victims.
And one we could work on making more of a certainty.
It's not like they couldn't be followed if they were dumb enough to go for it.
Once there...voila! A new targeting opportunity...get to include their new found "friends".