Fear of backlash, nothing more. They remember very well the aftermath of 9/11 and they dont want to face that again. The number of muslims who genuinely oppose this on principle is minuscule — you could count them on two hands.
My freind married a woman from Iran. I was invited to dinner at her parents house (who were muslims of course). I had not met them before, and was mentally preparing myself to be very respectful, and bring up nothing political, or religious.
I was stunned when I entered the house and upon meeting the father, the first thing he does is offer me a shot of jack daniels. We sat and drank jack daniels, together (he did too). They had a traditional Iranian dinner which was quite excellent. Since everyone was loosened up, we had quite an excellent conversation about politics and religion.
Afterwards, they gave me a practical life time supply of Saffron brought over directly from Iran...I guess the stuff is practically free there. I love my saffron rice!
So the lesson is, there are alot of good muslims, they are just people after all. I used to play chess with another Iranian and have known a few others over the years. It seems that most of the Iranians who fled the country during the revolution are pretty awesome people. The problem is there are enough bad ones to be a huge problem.