Evidence Obtained by Threatening a Teenager with Being "Gang-Raped to Death" Permitted in U.S. Court

The short version:  The US military arrested a Canadian kid for throwing a grenade which killed a soldier and put him in Gitmo for seven years.  He was 15 when he threw the grenade and is now 23.  He is being tried as a child in military court for war crimes using evidence obtained — and here’s the key part — by an investigator who threatened to have him gang-raped to death if he didn’t talk.

If this is a war, then why is he being tried?  Grenade throwing is normal battlefield activity, so in a real war he would just be another soldier doing soldier stuff.  If this is not a war, why is he being tried in military court?  Murder is dealt with capably by civilian judges, and guilty murderers are appropriately punished.  And regardless of what this situation is, why the hell are we using evidence obtained by threatening to gang-rape a teenager to death, however guilty he may be?

First let’s get our story straight.  Then let’s get our act together.  Then let’s go after terrorists legally, fairly, and proportionately when we actually have some remotely respectable moral high ground.  Because this is disgusting on every level and absolutely not justice in any sense of the word.