Although no one knows the exact number (or if they do they are not saying) of “Americans” who have tried to join or actually made it to ISIS it is estimated to be well over 180.
When they are captured in the US, they are charged with various crimes such as conspiracy to provide material support to ISIS, helping recruit for ISIS and/or conspiracy to provide material support to a designated foreign terrorist organization. They are not charged with treason. In fact, almost no one is charged with treason even though to you and me it sure looks like treason.
Take John WalkerLindh.
Caught fighting with the Taliban against us and later participating in a prison uprising of Taliban prisoners that resulted in the death of an America he was sentenced to 20 years without parole in a plea bargain charged only with supplying services to the Taliban and carrying an explosive during the commission of a felony. That means he cold walk in 2019. Wait, I thought it was without parole? Well, that’s the Government for you. This is because, although there is no parole under federal law, his sentence could be reduced by 15 percent, or three years, for good behavior.