Why do students use the passive voice in writing? I'm sure they use it very seldom when talking to their friends, so their overuse of it in writing must be an attempt to imitate the (perceived) dullness of academic prose, as they have experienced it in textbooks, etc... Academic writing, to the person learning such writing as a foreign language (essentially), appears pompous, dull, impersonal, agentless. All that "It could be argued that..." Freshman comp is an attempt to teach the students to write for subsequent college work, so there is an inherent contradiction between telling students NOT to imitate that insipid style, when that style is in their heads already and is the perceived goal of the course. In other words, they think WE write like that (as sometimes we do.)

I used a composition book once that was written in an atrociously tedious style, in a a style you would never want the students to use. It was a primer in dullness. And to teach them to write in Spanish, no less...