Because they're being lied to. Though "lied" may be too harsh. Dissemination of faulty or biased information isn't always intentional.
What? You don't think the xenophobia that the average American has towards Islam might have a little to do with it? 1 Geez, the guy is accused of attending an Islamic school like he spent a year in a terrorist training camp. 2 We send troops to Iraq and Afghanistan without even training them on the culture of the people they are supposed to be helping. We have politicians who probably don't even know what the Qu'ran is.
Islam is the second largest religion in the world, and it's growing faster than Christianity, so maybe the US should stop treating it like something evil.
1 Depending in what country the "school" is in, its the same thing.
2 That is a big problem and why the US is public enemy number one in their eyes. Our people do stupid things because they don't know the culture over there. They are not dealing with white christians over there like in wars of the past. They are dealing with a people, culture and religion that is totally different from our own.