like HLukas, mine too are on the left, otherwise the bayonets would hit the hats when held over their right shoulders.
Macruran, what you said makes sense to me.
.....maybe when aiming using their right eyes, its bothersome to still see the bayonet when their left eyes are not closed (but like Gordon says, those older type bayonets are fitted into the barrels, so it wont be there in the first place, when aiming).