The reason Pakistan will never fight the case of Khulbushan internationally is because they can't prove he was doing terror activity. And they will get in a fix if Iran tells he was in Iran & was abducted by Pakistan intelligence.
They will have no choice but to release him, which will be a big blow to the Gov.t & establishment in front of their public. Every country does spying, even that will back fire if the person is syping from outside like the case of Khulbushan. They can only file the case against that country in this case Iran, which Pakistan has no balls to do. Only if a spy is caught in their own country they have the right to prosecute him/her. But for spying also they are released if the country makes it public.
Pakistan never come to claim their terrorist. We are claiming Khulbushan because we know he was not involved in any terrorist activity. We show every Pakistani terrorist to the UN, ICJ & to be interrogated by any international institutions. This is the big difference between India & Pakistan. We always prove the Pakistan's involvement in India, but Pakistan only brainwashes it's public without even declaring its proof by their own media. Only crying is done to fool public.