Because its a poor practice that will bite you often, when you can least afford the consequences.
The reason you don't ever turn the motor backwards is because of the cam chain, tensioner, and chain guides assemblies. They are designed to maintain consistent control in one direction of rotation only. When you turn the motor backwards you transfer the precisely controlled slack half of the chain loop over to the opposite chain guide, which applies force to the tensioner and guides causing the tensioner to compress and the guide to bend. When you resume turning the motor in its intended direction it may require several revolutions for that assembly to restabilize. And who's to say if it might ever be the same again...?
The unique shape of the guide on the slack half of the loop promotes binding if the chain ever moves backwards. You can feel that chain and guide binding up and applying excessive force when you turn the motor over backwards... it turns over with a LOT more drag. So why in the world would someone want to do that?
Precision works require sound work habits. And I'm a very outspoken guy who has already learned these things the hard way. No offense intended, I just cringe when I encounter folks doing something wrong.
EDIT:
Another reason for never turning a motor backwards is the shape of the camshaft lobes. Those are profiled to operate in one direction of rotation only. The ramps on cam lobes are gradual on the lifting side of the lobes, while on the closing side of the lobes those ramps are much steeper. The valves are gradually accelerated to a full open position, held there for a period of time, then dropped closed as quickly as possible on very steep ramps. The cam followers and lobes can both be damaged (they are forced into a bind, high spring pressures multiply mechanical forces for this binding condition) when a person forces the followers to climb the steep ramps on the closing half of the lobes, which happens whenever a motor is turned backwards.
