What torque? A derailleur/single speed/fixed drive does not apply any torque to the axle, apart from the trivial amount caused by friction in the bearings. (Bearing friction would cause a similar trivial torque on the front axle also). The torque from the drive sprocket is applied to the wheel without the axle being involved.

In cases where the axle does have significant torque applied (coaster brake, drum brake, Rohloff 14 speed hub) there is a "torque arm" which is attached to the frame. This transfers the torque to the frame and prevents the axle from rotating in the dropouts. Three speeds don't generate much torque and so the flatted axles suffice.