The electrical office pops that in to clear themselves (cynically) of any liability over wire strikes. Instead of taking a duty of care position and adding balls to likely spans, they write a quick "thou shalt not" and blame the pilot without accepting any duty of care. Same sort of regulation as "Crashing is not authorised"
I think the testimony reference the dropping and powerline situation needs to seen be in context. One side is trying to suggest that the other side may have deployed assets differently to fight the fire at the beginning, and the other is trying to justify decisions made in the heat of the battle with little available info in a rapidly changing and highly demanding environment. An environment in which they are the subject matter experts.
Water drops around powerlines can be perfectly safe. Distance is the key, and for once they could leave that judgement to the pilots as they are nearly always more experienced than the Air Attack guys who are the fire experts.