It's illigal in most races, because it's like cheating. If you hit a barrier, you'll have to wait for the turn marshall to right the car. So, in a way you're being penalized for bad driving. Going in reverse would basicly fly in the face of that.
Oh, and no one wants to hit a backing car.
Tamiya 416X/ Tamiya TA-05 IFS/ Losi XXX-SCT/ HPI Baja 5SC/ Tamiya M05/ Tamiya F103/ Tamiya F104/ Team Associated 10R5O/ Kyosho IGT2/