Also, Marcus has a perfect excuse for not using the transporter to send a "police patrol" after Khan. Namely, we have no evidence that the unit could perform the return journey!
The device used by Khan was left in the wreckage of his aircraft. How could he activate it for a return journey? How could anybody back on Earth target the heroes and their captive for a beam-back? Even if Kirk took another unit with him when beaming to Klingon space, how could he establish return coordinates? And how could he be certain that Klingons didn't capture that second unit after the heroes beamed out but before the self-destruct charge hopefully fired? Too many problems there for the attempt to be practical.