Only radical Muslims would consider rioting a rational rebuttal to descriptions of Islam as violent. What other religious group riots or issues death threats after it is criticized? It is precisely because Christianity is so tame that Western liberals often feel safe to lampoon its history as violent. They wouldn't dare level similarly harsh criticism of Islam.
One of the unstated reasons for hesitating before calling radical Islam violent — the reason the fog of political correctness thickens around it — is that it does contain elements of violence. Western society falls silent lest its criticism of Islam result in an explosion of anger validating the criticism.
This amounts to nothing less than surrendering liberty to a violent minority group, and the beginnings of the tacit imposition of oppressive sharia upon a free society. Why is this tolerated?