The problem with not intervening is that it doesn't work unless every other actor in the region agrees not to intervene as well. Russia likes Assad because Syria gives them influence in the region. They're going to give the Syrian army aid and resources whether the US is involved or not. The US got involved in this ethnic conflict in part because they generally don't want Russia to gain any more influence in any part of the world. So they try to give equal amounts of aid and resources to the Syrian opposition. One side makes a move, the other side makes a counter move.

Whether or not you agree with those actions, or the whole system, is a matter of politics. I'm not condoning anything, just trying to explain the potential reasoning behind the decisions. I do hope that radical islamism runs its course in the same way that communism did. One big complication is that there are a lot more foreign powers involved in the Middle East than there were in the Soviet Union. It could be harder for them to find their own way while being manipulated by so many other hands.