"I use only one pan."
Translation:Mi uzas nur unu paton.
I would have explained it as "because pato is the direct object of uzas". That is, it answers the question "what is used" and not "who is doing the using." It's "receiving the action" of uzas. I see what you're saying, though. If you thought nur was a preposition, then the sentence might be a little tricky. As you said, it's not a preposition.
I write in some detail about prepositions in a recent blog post. If you're tempted to call nur a preposition, I would encourage you to read it.