"And he grabbed you by the hand
And he pulled and pulled
As someone pushed him and he fell
On the front page of America"
-- Spirit of the West
In al-Zawahri memoirs, it seems, he justifies attacks on Western civilians simply because Western countries are supported by their own people. Since people choose the governments and support them with taxes, the people are therefore accountable for their governments' actions.
It kind of puts two long-standing American institutions - not voting, and cheating on taxes - in a new light.