But you do see riots and violence over all sorts of Christian agendas. They just don't get as much press, as Western sects causing trouble around here is nothing new. And, as long as you're bringing up the example of the Mohammed comic, didn't that riot happen after the paper that published it declined cooperate with the requests of the imams of the region? So, it's not like the Muslims that rioted just read the paper, stood up and said, "Someone's gonna pay!" They were worked up about it because it became a point of formal protest and the concerns of their entire community (at least from the rioter's perspective.) Also consider that Muslims as a whole have had a big bull's-eye painted on them for more or less the past decade (or two) - that has to wear on you after a while. (Being slightly on the conservative side, haven't you grown sore of the constant Repub-bashing that's been so popular in the same span of time?)
I believe the same kind of thing has a way of happening here in the States when folks like Fred Phelps get up on their soapbox around here. The only difference is, everyone understands the nuances of neo-Christian politics around here can see that the small fraction of Christians who back up people like Phelps are the minority, are acting out of earnest (arguably misguided) concern, and are not representative of Christian attitudes or agendas at large. Want a non-secular example? PETA, gay rights, public schools, free trade, emissions controls, logging protesters... the list goes on. When people are unhappy and then taunted and ignored long enough, they revert to their basal instincts, and get violent. That's an attribute of people; not Muslims, not facists, not treehuggers.
I think the reason you don't see Muslims policing their own is because they don't have the means. Most Muslim nations are theocracies where the bulk of the populace takes relatively strict orders about their conduct, particularly about their political actions. I'm not trying to be standoffish, but I think you're outright wrong that the Muslims community didn't try to curb those riots; I remember hearing a lot of Muslim protest on the TV news and in the papers condemning the riots. It didn't receive much press until the imams also called for an end to the violence, but well before then I remember seeing quite a bit of "man on the street" interviews with local Muslims that were pretty much all of the bent that it was an inappropriate response, with varying shades of "but the paper started it."
I believe the important part is that Islam has a different structure and social dynamic than Christianity. As far as I understand, the Muslim clergy is much more uniform and closely tied than the governing bodies of the myriad Christian sects. Unless I'm mistaken, Islam is closer to single sect, like the Catholics, than it is to being an umbrella term like Christianity. That is, all formal Muslim practice falls under the authority of one group of clergymen, unlike Christian offshoots which mostly just share their scriptures and little else. As such, Islam comes off a bit more like an oligarchy - it isn't as adaptive or progressive as Christianity because there's a consolidation of authority among a few fairly permanent leaders. Many Christian churches are small and accept little dictation from whatever governing bodies they belong to; as such, the whole of Christianity is quite adaptive in it's stances on societal issues, since if one church is saying things that the congregation doesn't like, anyone can just walk down the street to a different church. That kind of option isn't really available in many Muslim nations, especially in third world and immigrant communities, as well as the first-world Muslim nations that are controlled by the church.
I think that if you were to look at some of the more cloistered Christian communities, you would find analogous instances of intolerance and violence. It's all a matter of scale.
~Joe