If every week a cartoonist draws Mohamed will Muslims bother to riot every week? Humans are lazy. What was once outrages can simply through repetition become a unsightly "feature" of those accursed infidels, much smaller in emotional affect and its impact on punishing a specific offender.

There are also more moderate (and westernized) Muslims whose feelings are hurt when they see a deliberate attempt to offend Muslims, even if they wouldn't be particularly mind if they encountered a drawing of Mohammed in say a history book or even a cartoon also featuring Buddha, Jesus, Jehova etc. Many probably wouldn't mind if drawing Mohammed and burning the Qur'an were forbidden, but wouldn't go out of their way to make that happen.