The powerline adapters works by transmitting signal over house wiring, and the wiring is effectively creating an antenna. So depending on which frequencies the adapter uses, will potentially radiate the same frequencies and the harmonics.
Wouldn't ethernet cables have the same issues? Does the increased power flowing through power lines somehow amplify the signal? Is that pretty much the definition of what a power amplifier in a radio does, add juice to a signal on an antenna so it gets broadcast further?
Does the amount of wattage going through the line effect how "loud" the interference is?