Because they enable it. People like to say that this is just the work of mentally ill people, but why then are Muslims so much more likely to be mentally ill and go killing people? The answer is that it can be justified within Islam.
Videos like the one you posted have no real significance to what is happening in the world because religion's real power lies within the cultures built around the religion and the 2 are not a perfect match. If you look closely at Islam there is absolutely no reason for Muslims to be offended by representation of Mohammed by non-Muslims, but they all consider it very offensive anyway which leads to the extremists using it as a reason to kill.
Problem with any religion is the Zealot retards who ruin it for all.
In regards to the cartoons. Saw a brief sound bite of a interview with a Cleric who simply said if you don't like the cartoons ignore it. Kind of funny when you think about it this argument about cartoons. There was that cartoon of Tim Jones NSR climbing into heaven and meeting a caricature of God. Interestingly there was no attack on the local Vancouver paper that printed that cartoon. Thankfully Christians don't follow the Jihadist mentality and have evolved past acting like Jack Asses.
I find it funny how again we discuss the zealot aspect of a religion and some nw this is twisted by someone into we are all racists against that religion.
Speaking of offence. I see that National Geographic TV is making a film about Jesus. And the horror…have a Muslim actor playing the part of God. Should we prepare for full blown riots?