Because the Qur'an tells them to. For Christians, God is a God of love; any judgment rests solely with the Lord. The more fundamental Muslims know that infidels must be punished in this world, and the next. It is a worldview that leaves no room for nonbelievers.
Originally Posted by Roelof
“To preserve the government we must also preserve morals. Morality rests on religion; if you destroy the foundation, the superstructure must fall. When the public mind becomes vitiated and corrupt, laws are a nullity and constitutions are waste paper.” – Daniel Webster, 4th of July, 1800, Oration at Hanover, N.H.