Historically, the terrorists tend to want heavy responses to their (relative) pinpricks. It makes the antagonized country/entity look crazy and it causes them to over commit resources in largely symbolic and ineffective (but large) responses. They're banking on further ham-fisted responses, not a slackening of resolve. The longer they continue to goad fruitless responses, the more they tie down their target and drain their resources etc. Eventually, it's not a matter of being resolute, it's a matter of eventually not having the resources to continue the flailing.This is the type of thing the terrorists want. It does not matter to them that America would go ape and probably nuke every inch of the area of Pakistan that Osama Bin Laden is hiding out in. They think that America does not have the guts to go for the long haul and they can wear Americans down to ultimate victory. And there is some reasons to understand why they think that way.
Just look at Osama's recent tapes. Cheering on the Democrats, saying every single thing (do we have to call them Islamomarxists now?) designed to reinforce further stupid mistakes by the current administration, and from types like Liberman etc.