Today's Times of India front page carries a report headlined, India irked by FBI caginess over Headley: ‘Why Didn’t They Tell Us When He Visited India In March?’
A sentence early in the article reads, "The growing disquiet in the government... was expressed by a senior official. 'It is very strange that the US did not inform us of Headley’s visit to India in March this year when, by their own account, he had been under their surveillance since at least September 2008,’' the official said, pointing out that India would have arrested the terrorist had it known about his identity."
Right there you have the probable reason why the FBI didn't inform India about the Headley case. The agency likely knew we'd do something idiotic like detaining the guy. Indian police have no notion of building a case before making an arrest. Our method is to arrest the suspect first, book him under a law that allows long detention without any framing of charges, torture him and extract a confession, then finally try and build a case from this confession and any corroborative evidence revealed during torture. In the end, more often than not, the case is thrown out in court and the suspect walks free. If he was innocent all along, he has served months in prison for no reason. If he was guilty, he's got off lightly.
I'm glad the FBI said nothing about Headley to anybody in India. Now there's a good chance the guy will actually get the punishment he deserves, presuming he did what he's been accused of doing.