i just think like seasame street on this one. Z is for Zombies! YAAAAAAAAAAAAAAAAAAAY! its our fucking letter of the day! lets whack it in a map pack! its because they are resurrecting old zombie maps. however this is only my opinion, something i never get brains for....except there was this one time...
JFK must have survived the events of "Five", seeing as treyarch put a picture of the JFK assassination at the end of the campaign, Kennedy must have found a way to contain the outbreak. if the thief does indeed take the bodies after they die (as stated in your OP) then possibly either (story wise) JFK Kills the Thief, OR, The Thief takes Kennedy to Aether and spares his life...or something.