The fascist giants of Mordor were invested in ensuring the Patriot Act got passed. They intimidated congress to vote for it. They used fear to get popular support for the legislation.

What they really wanted from 9/11 was a declaration of martial law and the ability to make people disappear. They hoped to create an uprising against the government for imposing the Act. Make sense? An uprising with a leader they would have supplied.

If such an uprising were to fail it would inspire people (through fear) to desire a more totalitarian government. A dictator they would have supplied.

Related: The Real Valkyries