The (far) left is not totally pro-Islam, in that they do not actually promote the teachings of the religion or its extremist groups like Al Qaeda. However, they tend to play down the Islamist terrorist threat (which is understandable to a degree because fortunately there really isn’t much terrorism in the US, and starting random wars as in Iraq actually increased terrorism), and much worse, support Palestinian terror groups.
They do this because in the current left-wing mindset, a group perceived as an underdog cannot really be wrong. Any criticism of any underdog is seen as “blaming the victim.” Violent minorities must always be right, or at least deserving of sympathy and excuse-making, while those seen as the upper-dog, the US or Israel (which they see as a European colonial project) are always critiqued harshly.
Even though most of the Palestinian propoganda is lies, and can be proven is such, the far left is not interested in listening to debunkers. They uncritically accept basically all anti-Israel rhetoric, because it goes along with their bizarre upside-down narrative of the Palestians as innocent underdogs who just want to live normal lives and the Israelis as the worst, most bloodthirsty European colonialists ever.