I have a slightly different theory but they could all be related somehow.  Ok so imagine this situation:
* 2 snipers at T spawn in de_dust2,
* both snipers are on the ramp behind the ledge,
* both snipers take opposite positions, one on the left, one on the right
* both snipers scope in to the double doors and only expose themselves as much as possible in order to align the crack of the double doors with the edge of the wall they're taking cover behind.

From the CT's perspective on the CT side of double doors, the sniper on the right will expose more of his body than the sniper on the left.  This means, at all times, from the player's perspective, his right side is always going to be more exposed than his left side in terms of getting behind cover.

Also, since the models are right handed, their weapon is going to be in their right hand.  After hearing, first hand, some Green Beret/RANGER/infantry stories, they tend to shoot the enemy's weapon out of their hands because that's where they tend to first look when encountering an enemy.  Therefore, it might just be human nature to shoot at the enemy's weapon?

In conclusion, I really don't know but I hope I may have tipped off someone that will eventually discover the reason!