No matter how much we can criticize Christianity or Judaism or any other set of beliefs, we have to admit their degree of intolerance will hardly hold sway against what Muslims can show us
So, first: They didn't even show Muhammad's image. Yet, it's sole mention is enough to drive radicals to threaten with death to the South Park creators and end up "like Theo Van Gogh" who, as the report said, directed a film criticizing Islam and was killed by a radical Muslim.
This sole fact is enough to show to what extents Muslim fanaticism can go. There are plenty of films and documentaries criticizing Christianity, I think the last and most successful was Religulous. Yet, as much as its creator, Bill Maher can be despised by the religious, he hasn't received death threats, less attempts to kill him.
Theo Van Gogh didn't have that much luck, the only difference being that he criticized a less tolerant and more fanatic set of beliefs.
Second: The website states that their statement is not a threat, but a warning. Does "if you do such a thing, you *might* end up killed, in the same way that another person who (according to our standards) disrespected what we hold so precious" doesn't sound like a threat? Obviously, it does.
Third: According to Ayaan Hirsi Ali (the writer of the film directed by Theo Van Gogh) the Quran calls for the assassination of anyone who criticizes Allah, the Quran or the prophet Muhammad. Yet, many Muslims don't act upon such call. That's a good thing, the same thing that many Christians do, so that they don't act following crazy statements in the bible. That kind of "cherry-picking" is ok, until the argument that "the bible is the word of God and, therefore, perfect" is made. Then it just sounds hypocritical.
Fourth: South Park creators say that it would be "hypocritical" to not make fun of Mohammad after making fun of almost everyone. Yet, they had the images of Muhammad covered. It's clear that they are afraid, even in a society in which freedom of expression is regarded as one of most essential rights. To that extent has reached the Muslim fanaticism.
Finally, there is one "what if": What if those weren't "real Muslims" and just Jews disguised as Muslims trying to make Islam look bad. Because, you know, threatening people and acting violent after Islam has been criticized is not what Muslims usually do. Islam is a religion of peace, right?