I was under the impression that bikes have a minimum weight in the rules and actual bikes are pretty much always under that weight these days? They then add little lead weights in strategic places the get them to the minimum weight (and help improve balance by placing the weights in the right spots). I'm not sure of the average added weight is, but you'd think it would at least negate some of the weight of the motor/battery (ie: the motor+battery weights 800g but there's 500g of extra average weight added to a bike, so they would only have a +300g bike..)

Note: I know pretty much nothing about racing but I have had that idea in my head for a while about the added weight. Maybe from a friend who told me his bike wasn't UCI compliant because it weighed too little?