Got it, didn't know about the front drive shafts being permanently connected.They are offering a FWD-only mode. Not RWD. There are physical disconnects on the rear axles that allow for this. The fronts are permanently connected so they'd create drag if they weren't always used for propulsion. For regenerative braking you also want the front motors, as they'll regen a lot more efficiently than the rear -- and it's safer to regen in the front, too. You don't want to cause the rear to lock up or lose traction, and that limits your braking/deceleration options there.
So, as much as a RWD "donut" mode would be fun, it really isn't practical. AWD for fun and spirited driving. FWD for eco.