I was out with a few friends last winter by the power lines in Atco. Seeing as they were in no way marked, I didn't feel it was illegal. Two guys parked off trail and we sat around and talked a bit, but it was just dirt anyway. No rare plants being squashed... Anyway, along comes two rangers... They gave us no trouble at all. They warned the two guys about being off trail and actually told us of some cool places to go drive off-road. So, I guess it's like getting pulled over by any cop or ranger. If you shoot your mouth off, you're likely to get in trouble. If you respect them and their jobs, they may just let you go.
I never really thought about those power lines much. However, routinely when I do the Quaker Bridge to Batsto run, I almost always exit by the fire tower and up on through the power line area. It's an obvious trail, but how is one supposed to know whether it's legal or not? I certainly wouldn't do it if it was.