Profiling & Subjectivity
This could be interesting* if anything happens with it.
I'm pretty sure that I, as a white male, with long hair, biker jacket, and an mp3 player blasting out Rammstein, would be able to look at Islamic history, paying careful attention to the Crusade era, without a problem. Hell, I could easily say I was researching for Assassin's Creed.
If my Asian wife (not a Muslim, but was brought up as one) was to look at the same material, it might be considered "extremist", as the Islamic response to the Crusades of Western Christendom is often used as an analogy for current Jihadist behaviour - in effect, Islam responded with violence to the Western Christian coalition in the Crusades, and they're doing the same now. The only thing that's changed is the method. The Crusaders use smart missiles instead of Templars, and the Muslims use IEDs instead of light horse archers.
Either way, the material is either legal, or it is not (whether some material should be legal or not is another conversation entirely!). Jumping on people who have chosen to view legal material for no reason other than "they might be thinking bad thoughts whilst viewing it" is not acceptable. What I might consider perfectly reasonable theological or historical research, would be another person's "extremist propaganda".
What happens if I view extremist Christian material? It does exist, as can be seen with websites about god hating homosexuals, to cite only a single example.
I feel the need to grow my beard long and enter an internet cafe (if I can find one!) with greetings of "Salam, brothers" just to see what happens. Though I'm not sure if I'm brave enough to risk being locked away and never heard from again, as seems to happen with depressing regularity.
*: note that I mean "interesting" in a subjective sense.... "terrifying" is also applicable here.