You're assuming there is even a transport leaving Organia and going into Federation space. Why would the Klingons conquer a planet and let Federation transports come and go? Seems the only way Kor will let you leave is either on a Klingon ship as a prisoner (but why when he has a whole planet to imprison folks), or as a pile of ashes. And a Klingon supply ship is not going to be allowed to traipse through Federation space and drop Spock off at a space station.
No, the quickest and least annoying alternative for Kor is to vaporize Spock and be done with it. He'd forget Spock's name 30 seconds later. Kor, however, don't have a reason to kill him once Spock undergoes the mind sifter and proves to not be a spy. Kor, to be fair, isn't a mindless murderer. So he puts Spock on notice, but the slightest infraction will result in the Vulcan's death. Until Spock is caught doing something wrong, he is now off Kor's radar.