I got the impression that the fins were for active guidance, the crash was reportedly due to running out of hydraulic fluid in the fin control system.
The fins are active in the sense that they are steered under flight control. That's why we have grid fins -- their hinge moments are much lower than planar fins at high aerodynamic speeds so you don't need as much servo strength to operate them. They are passive in the sense that they are like any other steering fin that requires headway in order to generate a working flow. Headway in this application means plummeting from the staging altitude at transonic speed. That's passive in comparison to RCS or APS systems that were used in Apollo on, say, the S-IVB. They can maneuver the vehicle irrespective of whether it's moving through a fluid.
It looks like SpaceX maneuvers the descending stage into about a 60-70 degree pitch-up attitude (well, a reciprocal angle-of-attack of 30-40 degrees, really) and holds it there. Theoretically you can control the drag if you vary the pitch angle to vary the angle of attack. Then at a certain altitude the engine is reignited and manages terminal descent under powered-descent flight control. The hydraulics come into play there again for thrust vectoring. Apparently this was the phase in which the Falcon 9 exhausted its hydraulic fluid.
Yes, open-cycle hydraulic systems are much simpler, but are generally predicated on the notion that you co-opt some of the petroleum fuel for fluid power needs and then dump it either into the powerhead or into the exhaust. Skydrol-type fluids are generally used where you can't use fuel. You can use plain old mineral oil if your fire and temperature constraints are not severe.
