Or simply pragmatic. Kruge still wanted Kirk to hand over the secret of Genesis (that was the only reason he beamed down in the first place), so he needed a means to blackmail him. The simplest way was to threaten Kirk's life with the very handily available dying planet, but Kirk would resist - so the second-simplest way was to threaten the life of somebody Kirk showed particular concern for.
None of this answers the original question of why the rest of the heroes didn't drag Spock with them "over there" when Kruge told them to move. But at that point, said heroes wouldn't have known that Kruge wanted them in that particular spot for beam-up. For all they knew, Kruge wanted them there for a mass execution - in which case leaving Spock behind would be doing a favor to the Vulcan.