I have this down the street from my house and I asked a lineman about it. He was saying they will never leave this on power lines, just on the telecom lines. The arborists work for the power company, because even thought the utility poles are jointly owned the power company has the highest risk so they are responsible for management.
They will always remove the wood from power lines because wood is somewhat conductive, especially when it still fresh because of the water content. Leaving it on a high voltage line can increase the potential for a short. They don’t bother to cut them completely off the telecom lines because there is no risk of shock but a big risk to line damage. If they damage the telecom line they have to pay high fees to the telecom company for repair and risk creating an outage. Also the lines are designed to bear a certain amount of weight, as engineer have accounted for natural burdens like trees, ice and animals.
Also the wood eventually rots after a few seasons and will eventually fall off on its own.
I was curious if what I had been told had any reference online, a quick search did turn up this post which pretty much says the same: