Preferential treatment? They tried to release McCain and he wouldn't go. He told them he would not leave until everyone imprisoned before him was released. I don't much care for McCain, but it took enormous courage to voluntarily stay in prison and be tortured more.
Funny thing, even while his committee under John Kerry as chairman, McCain refused to accept the evidence of other POWs left behind. There are witnesses who saw them shredding the evidence.
What they didn't count on was an investigator coming forward now with mounds of evidence.
You need to read more about the record of Senator John McCain in pursuing the interests of the men left behind. He has been a real self-serving snake. There is hardly any plausible explanation for his actions and attitudes toward MIA/POWs, other than he is genuinely emotionally unstable or a Manchurian candidate with skeletons in his past POW closet to hide.