Mr. Airline being a jerk

If you had a job at an airport shifting luggage around from plane to plane, you wouldn't want to toss around 100+ pound bags either. It's not about the total weight on the plane, but more of a union-mandated rule to protect those workers from getting that one bag that's secretly full of rocks and throwing out their back trying to get it on a plane and being unable to work anymore
Edit: fixing autocorrect issues