That makes more sense, if the fronts are regenerative, that's where most of the braking force comes from so I can see drums being sufficient in the back. A lot of lightweight motorcycles (especially classic styled ones) have rear drums.

The only thing I hate about driving a front disk/rear drum car is how deep the brake pedal is before you feel it activating the brakes. Going from responsive brakes to "oh shit they aren't activating yet..." panic for a second can take some getting used to!