You're assuming that they have access to all the information that you do. (Which they may not, depending on their circumstances.)

The information to form an opinion on ISIS is provided by ISIS, this included but is not limited to propaganda videos featuring gruesome executions of civilians and pows, a magazine that features the best ways to kill gays and images of infidels being killed, and slave price lists.

I ask you a question: What's the difference between a Conspiracy Theorist and a guy who joins Daesh? They both share the defining characteristic of having a completely different view on what information is trusted and what isn't.

One is a nut job that believes aliens have taken over the government and sits in his house to wait for them to break down the door.

THe other is a nut job that believes they are gods soldiers and breaks down doors to rape 10 year old girls and behead their fathers.

It's comparing a stun grenade to a missile.

And another question: Even if they did know 'what they were getting into', why does that mean that they must always be doomed for it.

Actions have consequences they support a group that murders and rapes. This is called being an accessory to a crime.

'Outlaw' Outlaw

They do not want to follow the laws of their home country and instead follow the law of a doomsday cult.

So what do they do? They stay. They got the message, once there, always there, never to return.

They stay and die or they flee and are executed by their former compatriots.

Your attitude, if expressed as policy, would result in people who might otherwise try to leave and not do it anymore, staying, for lack of options.

They can come home so long as they go to prison.