i think its because if you have a bash it still might fly off. Shock loadings are a strange and unpredictable thing and much much worse than just drag forces from air resistance being externally mounted.

Id imagine it would hurt if it came off inside a car and bashed you in the face. Thats why everything has to be properly mounted at most tracks.

Plus Id imagine the bloke following you in the westfield wouldnt be too chuffed if he got a camera in the face at 100mph!!!