If someone travels to an area where ISIS is active and we know that, then isn't that kind of enough to go on as evidence to detain them until it is figured out if they are a threat?
We are not talking about someone going to a random country and detaining them upon arrival. I am saying that people who are traveling to Syria and Libya, known hotspots for ISIS activity and regions that pretty much nobody has anything to do outside of aid workers, should raise a red flag.
If you have an honest reason to travel there as an aid worker or journalist, you are registered as such and exempt from it.
You are talking about innocent until proven guilty, but we lock people up all the time before they are convicted, because we weigh the possible consequences of having them free in society. And I think that in these cases, those consequences can be so bad that we have a good reason to prevent them going back into society.
We also arrest people if they are suspected to be about to carry out an attack. We wouldn't we? Would you rather wait until they set off the bomb, or catch them before?