And why not? What has been done to disabuse them of their jihadist ideology? From [1] UPI:

"We've already had instances where we know that people who have been released from our detention have gone back and have become combatants again," Rep. Porter Goss, R-Fla, chairman of the House Permanent Select Committee on Intelligence told UPI.

"I do in fact have specific cases," he said, when pressed for further details, but declined to say more.

A defense official confirmed to UPI several such cases had involved Afghans released from Guantanamo.

"At least five detainees released from Guantanamo have returned to the (Afghan) battlefield," said the defense official, who requested anonymity.