That said, a glance at the horizon tells one that the line isn't on the same level, but the tail sticks up quite a bit. The general rule of thumb is that so long as the power line keeps climbing in the windscreen, it's good, but how far it climbs is also important; it can still catch the tail.
It's usually better to go over lines than under. Going under is generally only advisable when it's impractical or dangerous to pull up, or when one has known adequate clearance below. Unless it's excess clearance that's patently obvious, I usually only go under power lines that I've walked and already determined from the ground.
If it's known power lines, that's one thing, but clearance under power lines can disappear very quickly, especially if obstacles materialize beneath them (fences, etc). Whereas power lines tend to dip between poles, especially over long runs (particularly so with illegal power taps that one sometimes finds in fields), clearance varies considerably from the anchor points on the poles to the low point where the wire sags the most, and it's not at all uncommon to find secondary wires, sometimes much harder to spot, strung below the primary wires.
They are not concerned with actual safety just avoiding being linked to a situation. Does it make sense? I don't think so, but their risk management does.
