Spin-stabilized bullets bleed their speed at a far slower rate, greatly increasing average terminal velocity and therefore lethality.

You're right– spin-stabilized bullets would have significantly less drag, thus would travel much higher, and thus come down with way more lethality. Honestly though, unless someone's trying to shoot the drone with a musket, all bullets are going to be stabilized somehow.

Nope. Maybe the idea was that an unstabilized slug will tumble and lose velocity quickly, after which it's just some junk falling from above, mostly harmless.