The hired mercenaries did not conquer Aleppo over the weekend, which was seen as a win-or-lose situation.

American Scofflaw

Worse, and despite the mercenaries burning the bodies of their comrades to prevent identification as foreigners, live prisoners taken by Syria continue to confirm that what is being sold to you as a popular uprising is in fact a foreign covert overthrow sponsored by the United States. 

Syria captured foreign mercenaries in the act of crossing into Syria from Turkey, and statements made by fleeing refugees confirm that it is the foreign mercenaries, not the Syrian forces, who are killing civilians.

From the US point of view, they cannot back down and they cannot lose Syria or the momentum for the invasion of Iran is lost.