The media keeps saying that Walker joined the "Taliban". According to military reports, Walker has proudly told his interrogators that he was Al Qaeda.
Big difference there.
The Taliban were the quasi-government of Afghanistan before the war and, officially, had no quarrel with the USA. Al Qaeda, on the other hand, was an organization of unlawful combatants with international reach who had a history of direct attacks on U.S. Embassies and on the U.S. military.
If Walker is determined to have been merely "Taliban" he might weasel out of this one. If Walker is determined to have beenm Al Qaeda, his goose is cooked.
Personally, I hope he's hauled before a hungry tribunal who already has a mouth-watering recipe for goose in mind.