My first thought? They’re hedging their bets. They may not have a lot of confidence in Assad’s long-term stability or his ability to conduct the war, and having more direct control over their own military forces give them both a vote over how the units are used, and the option of pulling them out to protect their own skins if things drop into the pot. Really, this is a throwback (or a continuation, maybe) of the old Middle Eastern* tradition of powerful men maintaining their own private armies to protect their own interests.
*Really, the tradition of everywhere except maybe Western Europe and the US since the rise of the unitary nation state….