It is treasonous to join ISIS in any way, which includes saying you've joined them. You have the right to say what you want about them and you even have a right to say that you support them but as soon as you declare that you are part of them you are now affiliating yourself with a terrorist organization and should be arrested.
People who are declaring that they will be joining a terrorist organisation must be arrested. My argument is that surely it is better to prevent the act before it has actually happened. The person has already formed their opinions and will already have the foundations in place to make them a risk anyway. Why would it be justifiable to let them continue and gain further harmful ideas and skills?
Let me use a comparison. Say that at a high school, a student declared that the next day, they would shoot up the school. Is it ethical to arrest them before they can shoot someone? Obviously, yes. Even though they have not commited any crime yet, not preemptively acting would lead to the unnessisary loss of life. One individual's life vs. The lives of many. In order to preserve the interests and lives of the majority, the freedoms of a few clear dangers must be barred. Anyone who says differently is supporting the loss of life with poorly reasoned inaction. It is perfectly ethical to act preemptivly in defence of life.
ISIS is an organization that has many horrible intentions. Among these intentions is the destruction of the Western world. ISIS is an anti-american organization that wants our nation to be destroyed. Therefore if anyone joins ISIS they are aiding the destruction of our nation. They are betraying us.
Definition of treason "the crime of betraying one's country". They are committing treason, which is a crime. So it is not preemptive.
Yes, ISIS is a clear threat to America and the more we can do to stop it, the better. It's America's fault that ISIS is even a thing, so it is our responsibility to clean up our mess and take responsibility for our previous actions. Being preemptive and preventative is not necessarily bad.