I mean I went by that picture and picked up a Wang Looowwwww sweet chariot, AC200000 to your faceeee... And then I see this picture and I think, that actually looks cool. Why cant we switch the arms up? I mean that would rock! TheSteelWolf
You'll find that throughout the BattleTech universe the vast majority of 'Mechs are right-handed, just like the majority of humans are right-handed. BattleMechs are not modular, and components cannot be swapped out easily. Modifications like that would likely take weeks, as the entire torso would have to be rebuilt and new weight-bearing parts fabricated for use on the other side of the 'Mech.
It's to show how awesome the tweaking is over here. We can swap the arms around on a 'mech. (Yes, that's actually my own internal justification)