The rear axle assembly does not 'rotate'.
It can't be a solid connection to the springs because the drive shaft/tube needs to be moveable when you go over bumps. The spring(s) need to move in this process too & this is why there is only one stationary mount on them.
To 'rotate' there would have to be a U-Joint in the rear.
It moves forward/backward-up/down when the springs change shape from road changes/the vehicles body movement etc.
