They let him play in their sandbox last time, up to a point.
This is true but the Hillary was running and the Clintons owned the DNC- you know Debbie Wasserman, Donna Brazil and the fundraising was directed to her. The DNC is still calling shots in the background but with less firepower. They can still pull the plug on Bernie in the background but it would probably just make the whole landscape even messier. Whereas last time it was a simple "take Bernie down and that will prop Hillary up" play.
What a mess. Food fight shaping up.