this confuses me also, i understand the devs are trying to keep the game playable and enjoyable for a large audience, even if it means not following traditional battletech rules/data....which is cool, but i do think if i spent time blowing out another mechs right torso, that mech should lose it's right arm
same thing happened to me, lost torso side completely, ran back to hangar, repaired, weapon on that arm still functioning fully, only thing i lost was protection on that side of the torso....
hopefully they will add the dependency of the arm to having an intact torso, and thus destroying a side torso completely will lose the appropriate arm and weapons...