Imran Khan is one of the most popular politicians in Pakistan. (His former career as a cricket star probably helps.) Like any sensible person, he is opposed to the Obama administration's practice of drone murder. This seems to marked him for harassment by the administration, as Glenn Greenwald reports: US detention of Imran Khan part of trend to harass anti-drone advocates (the Guardian)
There are several obvious points raised by this episode. Strictly on pragmatic grounds, it seems quite ill-advised to subject the most popular leader in Pakistan - the potential next Prime Minister - to trivial, vindictive humiliations of this sort. It is also a breach of the most basic diplomatic protocol: just imagine the outrage if a US politician were removed from a plane by Pakistani officials in order to be questioned about their publicly expressed political views. And harassing prominent critics of US policy is hardly likely to dilute anti-US animosity; the exact opposite is far more likely to occur.
But the most important point here is that Khan's detention is part of a clear trend by the Obama administration to harass and intimidate critics of its drone attacks. As Marcy Wheeler notes, "this is at least the third time this year that the US has delayed or denied entry to the US for Pakistani drone critics."