Same reason my 550lb glider and a 100-ton B-52 are both "airplanes." It's a general word that encompasses a lot.

The problem isn't so much the use of the word "drones" but the lack of any further qualifiers on it. Sometimes it's evident from context (an article about military drone strikes isn't talking about a 2lb quadcopter, for example) but sometimes it's not, and I think writers need to be a little more careful with it.

The word "drone" means an aircraft without a pilot. That's been the case since militaries began testing anti-aircraft weaponry with unmanned aircraft, around World War II. The reason it is used specifically for small RC multirotors (as opposed to RC fixed wing planes or single-rotor helicopters) is that more specific terms like "multirotor" or "quadcopter" are less well-known.