"drones are a very serious problem for airplanes"

Can we get an elaboration on this? I understand pilots being very skittish about things moving anywhere too close (as in "if I can perceive it, it's too close"), but is there a brighter line between "objectively problematic" vs "hysterically paranoid"? I assume the Parrot Bebop drone I want is over 250g, and don't see the reasoning for registration when I'm not stupid enough to operate it anywhere close (however construed) to a potential manned aircraft flight path.