Suppose a 250g drone fails mid-air, hypothetically at a height of 100m, and falls freely. At terminal velocity, it's a lethal projectile.
Have no illusions about the damage that can be done by small things falling from a great height, or travelling at a high speed in general. The 250g weight is entirely reasonable -- I would actually claim that it is quite generous.
Even in the worst case of a quadrotor - which can't glide or autorotate or fly with a missing rotor - a 250g one is not going to become a lethal projectile at terminal velocity. Even the heavy batteries from larger multirotors seem to survive a 100m fall uneventfully, they just make a soft thud when they land. A softball weighs something like 200g and getting hit in the head with one of those at 85 mph doesn't kill you.
In order to get a 250g rock up high enough to reach it's terminal velocity before hitting the earth again, you'd need some kind of launcher (e.g. catapult, trebuchet, Slingshot, etc) which are regulated. For example, New York made Slingshot a Class-4 misdemeanor.
Perhaps in some urban areas, but I seriously doubt there are federal regulations about throwing rock projectiles into the air with no intent to hit or harm a person.
FAA says that pilots may drop objects from airplanes, but puts the onus on the pilot to ensure "no hazard is created". And, once the object touches the ground, it becomes subject to local laws. So dropping a rock onto someone's property could be quite a few violations depending on the outcome and the property.
Have no illusions about the damage that can be done by small things falling from a great height, or travelling at a high speed in general. The 250g weight is entirely reasonable -- I would actually claim that it is quite generous.