|"Is it an ecology-minded piece or a "free" light show?"
I'd say it makes something visible, either way. The idea that the pylons 'leak' electricity is an assumption (and even if true, by the sounds of it not the cause of the bulbs lighting), but the fact that bulbs will light up from mere proximity to pylons is information nonetheless. Electricity is spooky, and hard to understand - hard to believe in, even. A field of lit bulbs with no wires running to them is a pretty strong statement. Also, nice to learn the term "mutual inductance" from slashdot.