"Strangely enough, only Islamist terrorists seem immune to tracing on the internet."
Or the FBI, Secret Service, NSA etc. don't publish their tracking methods?
Tracing IP's isn't rocket science to the tens of thousands of network engineers around the world, and it's by no means a secret. Tools and docs are posted all over the Internet, and have been for many years.
Law enforcement agencies (which employ network engineers and computer security folks for precisely these purposes) have the benefit of legal access to ISP logs via warrant or subpoena. Those are key to busting Islamist scum in the act.
Speedy logistics and international reach are the other key factors. One would need to act fast, and often to gain access to foreign ISP logs (one would thus need the cooperation of foreign governments and foreign law enforcement).