Simple enough. They attacked our way of life, what we are, not merely a few buildings and those inside of them. If you (as a terrorist) can cause the USA, the shining light of liberty since 1776, to impose martial law or a severe set of restrictions on the freedom of its populace, then you will have more effectively destroyed America than if you had destroyed several cities with nukes. America is not just its people or its material assets - we are an idea, the idea of freedom and liberty. Martial law or similar restrictions will destroy that idea.
You see, that's where you're dead wrong--confusing symptoms of the disease for the disease pathogen itself.
America has become increasingly filled with godless, self-worshipping, undisciplined people who either despise all authority because it puts a crimp in their self-worshipping style, or who demand that government treat them as victims and provide for their every physical or psychic desire. These people understand only half of the calculus of freedom: the self-oriented side. The other half--the God-seeking side--they reject and hold in derision.
Each American begins to lose his liberty the day he rejects God and the necessary checks on personal behavior that traditional Judeo-Christian religion teaches. The heathen will rage at that truth. OWK will surely smell a spectral "boogerman" lurking behind it. But truth is truth. Restrictions on liberties inevitably result when we reject God.
Rage and howl. Rage and howl.
HOW DOES THIS SERVE THEM?