It would be interesting to investigate just what the relationship actually is between President Assad and IS. As you probably know the US Treasury has released details of growing evidence that the chief financer of IS through its illegal oil grade is actually the Syrian government, which has long been suspected of bolstering IS. And, further, that Russian financial concerns as well as Syrian business men, are facilitating the money laundering.
Why does it do that? Pretty obviously because they need the oil, which they no longer have control over. And if IS uses the money against Assad's real enemies, then so much the better for him for now