Ask your questions, don't assume they can read your mind. That is a good amount of hard work for a neighbor to take on for no apparent good reason. Also they had no intention of hiding the fact they did it as evidenced by stacking the debris in their yard. They are either insane workaholics, or had a good reason from their perspective to cut the branch down. Does not excuse them for not communicating. Maybe they think like you, and inferred it was ok, since you never stated they could not do it.