In several countries, including European countries, there are public worries about citizens travelling to Syria. Although those press reports are not always precise, what is usually meant is people travelling to territory controlled by Islamic State with the purpose of taking up residence. Some are arrested upon return.
What are they arrested for? Does any country in Western Europe prohibit its citizens from travelling to an area controlled by an organisation designated as terrorist? That would leave journalists travelling to IS- or PKK-controlled areas in violation of the law, so I suspect not. Joining an organisation designated as terrorist is often prohibited, as is joining a foreign military. But does taking up residence in an area controlled by such an organisation constitute either of those? Or do legislators suspect anyone returning from Syria to have actively supported the Islamic State (para-)military forces and/or be preparing terrorist attacks at home?
(This question is hypothetical. I am not asking for myself. I am not even asking for a friend.)