My experience is that people who do this intentionally are unethical. Not just the act of printing the e-mail. The fact that these are people that prefer to not leave a trail. Unethical people behave differently in almost every way because they are concerned first with covering their own ass and second with getting the job done in the best way.
I'm sure there are people that are just idiots, but I'll bet you find that more of them have ulterior motives.