Crudely defined, the US has no interests at stake in Syria, and the Obama administration was never enthusiastic about overthrowing Assad. It dithered long into the initial repression, and once events forced it into declaring a position, the gap between aspiration and action was never bridged. The foremost US concern — keeping weapons from falling into hands that might turn them on Israel — starved the most effective fighting forces of support. The Free Syrian Army withered, a vacuum was created, and the Islamic State of Iraq and Syria (ISIS) gained.
Return to 'stability' might sound attractive after a decade of neoconservative chaos, but proponents forget that ISIS emerged as a consequence of US support for despotic regimes across the Middle East and North Africa.
By allying itself with the regime, the US will be making itself party to the suppression of the Syrian people's right to self determination.