Steyn is not being sued. Amagazine which published an excerpt from America Alone is being sued.
You are right, of course. It is Macleans that is being sued, not the author. The Arabs have used the same tactic in suing, inter alia, Cambridge U. Press for the book Alms for Jihad, and Blackwell Publishing for an article written in Africa Confidential. The aim is to scare the deep pocket publishers. Publishers have the money and can be frightened; most writers are broke and aren’t easily intimidated.