What's the difference between a 240lbs rider on a 50lbs bike or 140lbs rider on a 150lbs bike?
What's safer? 40 lbs of battery on a 30lbs bike or 60 lbs of battery on a 120lbs bike
From a safety point of view, I'd rather be hit by 200 pounds of person instead of 200 pounds of bike. Even if it were 180 pounds of person on 20 pounds of metal, I'd prefer that over being hit by a 200 pound bike.
Also,
Consider if we limited the weight of anything legally called "bicycle" to 100 pounds. Now if a rider weighs 300 pounds, we'll be looking at 400 pounds.
What if the same rider can legally ride a 250 pound vehicle? We'd have 550 pounds, instead of 400.