According to this BBC article, Theroux has interviewed members of the Klan.
How does he consider the Phelpses to be more extreme than the Klan? For one, they are probably as anti-homosexuality as the Phelpses, and added onto that they are racist and have violent history. Also, personally wouldn't consider them to be among the most intelligent of people.
They are devoid of rationality, not unlike any of the extreme muslims.
If they protested a funeral of a soldier that was in my family I would take their signs away from them. If they struck me in the least I would beat them to an inch of their lives.