Why do you need foolproof evidence of blame in Syria's chemical weapons?

A lot of people are demanding foolproof evidence that the Syrian government was at fault for the chemical weapon attacks.  I wonder though, how necessary is it to show that the Syrian government did it?

The Obama Administration has indicated that it is planning to use force to go after those weapons.  This is a key point in using force; the US is not trying to displace Assad directly, after all.

If the rebels were snatching chemical weapons from government sites, then using them on their own people to garner press and outside involvement, it might as well be a pro-Syrian goal to remove as much of these weapons as possible from the region, making it that much harder for the rebels to grab them.  It comes down to logic: If Syria doesn't plan on using these WMDs, why keep them, only to have them fall into the hands of rebels who would use them for propaganda purposes?

So you see, it doesn't matter who used them, because the goal here is to remove chemical weapons from the war and the region altogether.  The fewer WMDs in Syria, the fewer casualties from WMDs caused by either side.

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PS. This doesn't mean that the US strike unilaterally, now.

What the US could do, is demand that the UN be allowed to go in and destroy (or remove) all chemical weapons on both sides, and quickly.  Without agreement, the US will therefore have a green light to unilaterally take out those chemical weapons.