1. They aren’t “pro-Islam” but rather they are frequently in a situation of “the enemy of my enemy is my friend”. Radical Islam is opposed to the western civilization, and dislikes Jews and Christians in particular – meaning they are opposed to something the “Left” dislikes.
2. The Muslim worlds is very left-wing in economics (as are orthodox Jews), and for the most part ignore racial and ethnic boundaries.
3. The western left is increasingly isolationist meaning they prefer not to be involved with all those foreign cultures (whom they consider horribly inferior though they don’t say it). Not wanting to fight Muslims doesn’t mean they like them – they want to ignore them. For example, many Brits were very isolationist in the 1930s, and didn’t object to Hitler until he tried to conquer their country.