The lesser of two evils is still evil. And who says that al-Qaeda is necessarily an improvement on Bashar al-Assad?
Al-Assad is the devil we know, who knows what or who al-Qaeda may invite into the tent.
And in the many-pronged assault on the existing regime in Syria, it is not at all certain which faction may end up in the position of “victor”. Al-Qaeda by no means has any kind of “lock” on supremacy.
AlQaida and other anti-Assad factions are also anti-Christian and have a recent track record of exterminating Christians wherever they find them.
In short, all choices are bad, but it's been a very long time since the USA was attacked by Alawites, or Syrian Christians.
Maybe if he 'rebel forces' could guarantee total eradicaion of the AlQaida affiliates first ~