That's not actually that's not true. ICE powered ultralights are weighed without fuel. The fuel limit is 5 gallons. So while the empty weight (in the regulations) is 254 lbs, the expected weight ready to fly is 300 lbs with fuel. Since the batteries don't have empty/full weights, it makes sense that they would increase the limit for electric ultralights to 300 lbs. This does not include weight added for safety devices in the event of catastrophic failure (eg ballistic parachute).
Also the main reason for weight limits (including fuel limit) is to limit potential damage in the event of a crash. This is through a limit on kinetic impact (in addition to weight limit there is a speed limit) and hazardous material (the gasoline).
So if we eliminate the highly flammable gas, then it's possible the battery weight doesn't really matter because a 254 lbs airframe will be physically limited in the weight of batteries that it can support.
Also the main reason for weight limits (including fuel limit) is to limit potential damage in the event of a crash. This is through a limit on kinetic impact (in addition to weight limit there is a speed limit) and hazardous material (the gasoline).
So if we eliminate the highly flammable gas, then it's possible the battery weight doesn't really matter because a 254 lbs airframe will be physically limited in the weight of batteries that it can support.