Why would they?
They have been kicking the rebels arse recently. What is their motive for gassing civilians? they want US bombings....come on man.
There's credible evidence they've used it in the past...who's more likely to have access to chem weapons, some rag-tag rebels who've been having their asses kicked recently or the government? Consider the breadth of the conspiracy for rebels to stage and kill their own civilian supporters. Finally, the intercepted communications provided by your fellow crazy in the OP link appears to seal the deal.
Then let's consider your one link to evidence...a somewhat disgraced former UN official, claiming hearsay testimony from someone is Syria...basically it is the weakest possible evidence, but it confirms your bias so....
All that being said, we shouldn't get involved and Obama shouldn't have redlined the matter and put his/our reputation on the line.