I think the 'evidence' is the problem.
Assad may be responsible - but trust in 'intelligence sources' is pretty thin on the ground - for obvious reasons.
In this particular case, the 'west' appears to have been itching for some red line in the sand to appear. Rather than suggest that Assad must prove innocence it would surely be more appropriate to simply show us their proof.
After all it must be pretty convincing if they are going to war over it?