Religious taboos and South Park

A recent episode of the Comedy Central cartoon South Park had an appearance of Mohammed, although dressed in a bear suit for modesty.  Here come the threats:

The posting on Revolutionmuslim.com says: “We have to warn Matt and Trey that what they are doing is stupid and they will probably wind up like Theo Van Gogh for airing this show. This is not a threat, but a warning of the reality of what will likely happen to them.”

In the South Park world this depiction is not particularly offensive, but Muslim culture prohibits all depictions, even complimentary ones.  The question:  do Islamic taboos apply to non-Muslims?  And the auxiliary question:  why does insulting most religions lead to (usually weak) letter-writing campaigns, sponsor boycotts, etc., but an infraction against Islam can lead to threats and violence?