Apparently, once Spock passed the enhanced interrogation, Kor figured it was less trouble to just throw him back.
I'm surprised this script wasn't held up by the seeming need to show a Klingon fleet at a time when the show had no Klingon ship miniature. Of course, as with the Antares in "Charlie X," in the end they simply neglected to show it.
Even as a young viewer, I could discern the severe limitations on STAR TREK's fx at that point. And yet I totally forgave this deficiency and felt I could ask for nothing more. Every little glimpse of the Enterprise was magic enough.
In his book by the same title, David Gerrord reports that his script for "The Trouble with Tribbles" met resistance because it called for a space station and of course the tribbles that would have to be made. He pointed out that they'd have the station miniature for future use, and it helped get the story greenlighted.
It would turn out that stock footage of K-7 is all they needed later, but that was still an asset that came out of "Tribbles."