>I don't buy into the idea that the Syrian government can claim some level of legitimacy by default and everyone else is subject to criticism for gathering support from other places....
The fact that the government is recognized internationally (see my first comment) DOES automatically give it a legitimacy in the eyes of international community. Unilateral support of rebel forces opposing the recognized government without UN Security Council sanction is generally viewed as an act of aggression and usually condemned, but I guess the US is so "exceptional"[0] the usual rules do not apply to it.
