Interesting bit of propaganda here. According to "US officials", Washington and AQ are on the same side in this one. How odd!
Wild guess: Washington believes that a good deal of "unconventional" violence (i.e. terrorism) will be necessary or advantageous in the overthrow of Assad, and that attributing it to AQ will be politically expedient in maintaining support back home. It will also provide pretext for intervention and/or escalation if and when it appears AQ will be taking over in Syria.
Nevertheless, it is a violation of the basic tenets of propaganda to afford the targeted group, in this case AQ, any favorable press, the opportunity to be on the "right side" of any conflict. It appears we are entering a new evolutionary stage in the history of criminal statecraft.