While members of the American Left make eloquent claims to having no prejudice against Islam the real truth is that only an apolitical, purely westernized Islam, that has no social, political, consequences, is acceptable to them. Therefore the fact that the people of Syria due to the pressures of the civil war have increasingly embraced the more political forms of Islam is viewed as anathema.
Furthermore most Westerners make no distinction between the various forms of political Islam. Thus the more moderate forms of political Islam such as the Muslim Brotherhoods, and the Turkish Justice and Development Party which accept many aspects of Western democratic political practice while attempting to be loyal to the traditional practices of Islam are lumped together with groups such as Al Nusra, Al Qaeda, Taliban, etc which reject democracy in principle.
This anti-Islamism in the West means that Westerners can only see the war in Syria as being a war between Islamic extremism and the Assad dictatorship. I suspect that many in the Left in fact see the Assad regime as the lesser of the two evils. The reality is that the human desire for justice and freedom in an Islamic world almost enviably takes an Islamic form. That Islamic form does not negate the fact that the armed struggle of the Syrian people against the regime remains a genuine struggle for justice and freedom.