In the US, prosecutors usually file higher charges, then allow the charged to plead down. If the result is a coviction for the correct level of crime, they did his job.
"the death penalty for juveniles is been abolished in the USA " and "he can't be executed".
Well, I think we need a lawyer for these. Many LA steet gang thugs are sent out at about 14 to kill their rivals. They are told they get one "free one" at that age. However, by the time the killer get near the injection table, it will 10 or more years due to automatic appeals and various stays of execution. We have many legal groups who do little else but file such appeals.
Also, I think the definition of 'juvenile' has been adjusted in the legislation you mentioned. I'm sure a 17 year old could be executed.
Still more, this is a military tribunal and is probably not resicted by the recent law you mention.
"In 2006, the US Congress passed the Military Commission Act" You should have no trouble finding the text to this act on the net. You may choose to read it.