Advocating to join a terrorist organization, or providing support for such, is illegal. Believing or saying stupid things is generally not illegal. Equating the two is nonsensical both from an intuitive and legal stance.
Terrorism is not crime. It's war. It is a grave threat to the criminal justice system to try and conflate the role of the military and the role of the police by treating (foreign) terrorists as criminals and not combatants.
Terrorism can be brought on by a state as well as by non state actors. Wikipedia defines terrorism as "the systematic use of terror, especially as a means of coercion." Conscription during any of the major U.S. wars in the 20th century fits this definition nicely. Many more people died from these than on 911.
I disagree with it too but I don't think your argument is good enough. They're arrested for joining the terrorist group which not only signals intention but means and possibly opportunity to carry out actions.
Your questions are about an action and a desire (or intention), not about ideas. Your last sentence was acknowledged and addressed by their post I think...
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