Houstonguy,
It's not just Rockhead. It's ALL left- wing Christophobes who do that. For another example, look at ultra- Christophobe TMN (TheMastersNemesis).
The other replies sum up pretty well the reasons why this is the case for not just him/ them, but for all ultra- liberal Christophobes:
1. Cowardice. The Left know that if they insult Christianity, generally the most they might have to worry about is verbal criticism from Christians, maybe a letter- writing campaign against them, as Christians adhere to the teachings of "Love thy neighbor" and "Love thine enemies." In contrast, they also know that if they insult Islam, it is extremely likely they will be made a target for violence from those practitioners of the so- called "Religion of Peace."
2. It's an East vs. West thing. They associate Christianity with the "Western" (In other words, American) worldview, which they see as evil, racist, and brainless. In contrast, they associate Islam with the "Eastern" worldview, which they see as beautiful and enlightened.
3. Delusion. They genuinely believe all Christians everywhere are evil, brainless, gun- crazy, racist hillbillies who are a danger to everyone, and genuinely believe all Muslims everywhere are sweet, innocent, peaceful little angels who would never dream of harming anyone. When a Jihadist group engages in a terror attack, the Leftists portray them as wonderful heroes taking a righteous stand against racism or Islamophobia or some crap like that.
4. Kindred spirits. The Left have much more in common with radical Muslims (i.e. Hatred of Christians and Jews, hatred of the US military, hatred of Republicans and whites, Love of pedophilia/ child molestation, a desire to see America as it now exists completely destroyed, a belief that violence is perfectly okay as long as it's "For the cause," etc.) than with Christians.