I read an interesting argument.
Islam is a military philosophy. An Islamic person is considered a soldier at war with all that is not Islam. If they leave the -faith- they become apostate. The penalty for apostacism is the same as it is for desertion under fire for soldiers; death.
There is nothing wrong with stopping enemy (by their definition) soldiers from entering your territory.
Fortunately for the rest of the world, a lot of Moslems are pretty crumby practictioners of Islam...