The reason for the far left == pro-Islam confusion is threefold:
1. Both are rabidly anti-capitalist.
2. Both dislike the United States.
3. Some leftists (ie Galloway in Britain) at least seem to be pro-Islam.
4. Liberal sentiment demands that fanatic suicide bombers be identified as the frustrated poor, held down by American global domination.
5. Both see Zionism and Israel as evil encroaching Western entities.
to be fair, I think most liberals support Israel, even if they don't support its actions in the territories. Plus the far right and old right are totally against Israel.