But comparing it to reactions to offensive material about Christianity is impossible; they just react differently is all.
The history and teachings of Islam differ drastically from the way modern Christianity has gone.
Defending the faith is still a core part of the Muslim religion - whereas Christian churches have been steadily pushing people away from those ideas. It explains why there is a difference.
As to why the individuals do what they do; well that is a personal liability. I suggest that there are those who would do the same if Christianity preached similar teachings in the modern day (indeed, plenty of them did in the past. And look at the Christian extremists - who foster much the same ideas)