I think the key thing here, in fastair's words, is: GOOD INTELLIGENCE.

The CIA/FBI/DHS don't - generally speaking - have good intel. That's why if they had received the threat stateside, they probably would have ordered the plane grounded. Think Cat Stevens.

Other world agencies, while not infallible, have had to deal with bomb threats much more often and, as such, have developed a method by which to assess the true risk of the warning. The Israeli and British Security Services, for example, do not respond willy-nilly to every threat. They've learned the pattern of credible threats and can put them into context.

Even though I don't have all the data, one of the most telling things about the threat's credibility was the flight number given in the original report - the SK codeshare value for the UA flight. No one would call in a codeshare flight number for a bomb...

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