Well not everything that flies. (Lets not get into the 'Birds aren't Real' conspiracy). Your average pigeon isn't a drone, nor is a parrot. However, those are biological things. So they have something piloting them (ah the joys of meatsuits). While some flying insects are classified as drones (see worker bees as example), we still think of them as insects/bugs. However, a non-biological that does fly and does not have a pilot residing in it? Effectively a drone.
Yes we have ancient RC helicopters and model airplanes. They are essentially referred to as drones now. Even more so if they transmit back telemetry or visual data and take remote commands. In this way, it is being corrupted by marketing, but it is very much what the language has vernacularly evolved to.

If you want to get technical, then by all means, whip out the documentation and point to the definition.