Let's say I'm the national law enforcement arm of a superpower government, and I discover that an operative of the world's largest, most dangerous terrorist organization is arriving by air for a mission that is unclear, but includes reconaissance and possibly participation in a major attack. My course of action would seem obvious: I would keep him under round-the-clock surveillance, monitoring all his travels and communications--meetings, telephone calls, Internet traffic--hoping to learn everything I could about his colleagues and their activities. I would only arrest him on arrival--jeopardizing a potential future intelligence bonanza on the off-chance he might have been supplied with a lot of advance information, and that he might actually reveal it before legal chaos ensued--under two circumstances: desperate need for a P.R. victory, even at the expense of future successful investigations, or lack of confidence in my ability to prevent him from slipping underground, disappearing to pursue his mission undetected.
It's hard to say which circumstance would be more appalling.