Stealing energy from radio towers or power lines
Back when I was in grade school I had a crystal radio set.  A crystal radio contains no amplifier.  The output signal is completely powered by what is picked up from the antenna.  I had around a 50 foot length of wire running out my bedroom window to a shed in the back yard as a antenna.  With that I could pick up a 50 kW AM station over 20 miles away quite clearly.  It was reasonably loud with headphones, a few k&Omega; impedance.  I hooked up a impedance matching transformer to drive a 8 &Omega; speaker with it.  The radio program was easily audible with my ear up to the speaker.  Sometimes I left it on at night to annoy my brother.  You couldn't make out what was said accross the room, but you could hear what sounded like distant talking, enough to be annoying if you didn't know what it was.

I can't say how much power that actually was, but enough to harvest and use by a low power intermittent device is possible.

As for loading the transmitter, that only happens in the near field.  The radio station I mentioned broadcasts at 1.03 MHz, so the wavelength is about 290 m.  For anything much beyond that, the power is already propagating irrevocably from the transmitter such that it can't see any loading.  Put another way, the transmitter has already been loaded with that power, whether you use it or let it propagate into space forever.  Since I was over 20 miles away, I was well past the near field.  My receiving the signal only reduced the field very slightly in the vicinity of my antenna.  As far as I know, there is nothing illegal about using a 50 foot wire antenna completely on your own property to receive AM radio stations.