Freedom of Speech under attack. The internal battle with ourselves.

In Virginia a 17 year old plead guilty to charges of using bitcoin to aid a terrorist group. He may face 15 years of prison time. The Guardian says :

A tech-savvy US teenager has pleaded guilty to using social media to aid the Islamic State group, a crime punishable by up to 15 years in prison, the Justice Department said.

I have to pose the question: What did this person do that was illegal?

Is it illegal that he expressed his opinion? Certainly supporting Isis would seem to be bad, but is it illegal for he to say that he supports them?

Maybe advocating that people send bitcoin to ISIL was illegal? Is it legal to send the money you have earned to radical Islamists?

Perhaps the terrorist angle makes it illegal. America citizens are entitled to freedom of speech. Unless it involves terrorists. If that's the case, no warrants needed the government can arrest and detain you and undefined amount of time.