Many times An Arm and a Leg
are lost in the line of duty. Sometimes a Life or Limb Decision
has to be made. No matter the circumstances its always the non dominant arm lost, even if it was done under conditions that would likely have a 50/50 chance.
The meta reason is likely that the actor simply can't perform without the dominant arm. This is also likely to happen in adaptions, as the character loses the non dominant in place of the dominant to allow the actor to perform well.