Taking away Assad's chemical weapons doesn't serve their purposes. They want to get their hands on those weapons.
We're supposedly intervening in Syria because the government used chemical weapons in the civil war. Meanwhile the Syrian rebels are making it clear that they couldn't care less about the chemical weapons.
Qassim Saad al-Din, a spokesman for the Syrian opposition military command, told the London-based paper Asharq Al-Awsat that the Russian compromise with the US regarding Syria’s chemical arms was reached “at the expense of the blood of the Syrian people.”
According to al-Din, “the only thing the West is interested in is protecting Israel and disarming the arms threatening it. The 100,000 killed in Syria do not top their priorities.”
Head of the Military Revolutionary Command in Aleppo, Abed al Jabar al Akhidi, said that “the international ploy to cancel the attack on Syria in return for the neutralization of chemical weapons is a dirty deal between Russia, Assad’s regime and the West.”
“Neutralizing Assad’s chemical weapons serves Israel, not the Syrian people, because the international community’s attempt to disarm Assad from his chemical arms stems from the public outcry and not from the killing.”
Setting aside the expected rants about Israel, the Syrian rebel leadership is making it clear that they couldn't care less about Assad's WMDs. What they want are airstrikes that will cripple his military capabilities.
Al-Akhidi says it rather clearly. Taking away Assad's chemical weapons doesn't serve their purposes. For one thing they want to get their hands on those weapons.
Reasonably speaking, you might expect the Syrian rebels to be upset at the delay. But this is a complete disregard for the foundational issue of the intervention.