imprisoned on Guam?
Is that because the extradition to the USA could be deemed illegal, so he's been put in a USA territory outside the regular legal system?
Guam is American territory. It has been American territory since the Spanish-American War of 1898. (The Spanish governor of Guam discovered that Spain was at war with the US when he woke up one morning to see lots of American ships in the harbor.) There was a brief interlude when Japan occupied the island, but other than that it's been American ever since. The biggest air base in the Pacific outside of Clarke AFB in the Philippines is Andersen AFB on Guam. During the Vietnam war it was a major base for B-52s and the like. Whenever things warm up in the western Pacific, Andersen gets dusted off and a few B-52s, B-1s, and B-2s plus KC-135s, KC-10s, and assorted fighters move in. The last I heard, a half-dozen B-2s from the 509th Bomb Wing were parked at Guam the better to irritate Little Kim in North Korea. (The 509th flies B-2s, and is the only unit in the world which has actual combat experience dropping nukes; as the 509th Bomb Group they were the boys who nuked Japan. The USAF sends the 509th out when they want to make an impression.)
Guam is American territory. Sorry to banish the tin-foil hat, but there it is.