Sure . . . let them actually have the wherewithal to travel anywhere in the Levant to create havoc there? maybe even attack Israel?
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.
So, perhaps they can stay in Germany where they will feign remorse, fade off the radar and then....strike?
Until somebody does something, not much you can do to them.
The history of surveillance over guys like that is not good. Note the Paris terrorists, all of whom raised their hands some time ago...and then were ignored.
The threat of being stripped of a German passport is enough. After that, they’re nobody. Maybe they can get one from ISIS. Sort of an instant “go to jail now” card.