How much evidence do we in fact have that the rebels gassed their own people in a false-flag operation? I had heard it was a possibility, Putin seems to think it's all but certain, but I haven't studied the issue in depth.
I think the problem is that there isn't much evidence either way. The Obama administration thinks it was Assad, as that fits their model of the conflict, where Putin thinks it was the rebels, as it fits his model. I'm of the opinion that you shouldn't go to war when the only reasoning behind it is evidence that you're fitting to your model.
Why would Assad use chemical weapons when he knows that the US is looking for a reason to invade?
He isn't an idiot.