Yes, inductively coupling to something increases the load on the primary (or any transformer would be a free energy generator). If it were using only the heat differential between the conductor and the air, like a Stirling engine, then no. In fact, if it acted to cool the powerline, it would probably increase efficiency, to a microscopic degree. And at that point, you'd get better performance from a solar panel.
However, inductively stealing a drone-portable battery on any practical time scale would be all but imperceptible on a powerline at 100kV carrying 500 amps (50MW).
If it's a UHVDC line (or any kind of DC), inductive coupling won't work at all.
Of course it's usable. First law of thermodynamics. It's basically electricity theft wether one does it directly or through induction. It's called a vampire drone for a very good reason.
