Prevent "I don’t know" as an answer

I see this phenomenon in online stores (especially Amazon) that allow users to ask questions of prior buyers. Some of the buyers respond with answers of the form, "Sorry, can't help", "I don't know", or "I'm not sure"

It's not uncommon for askers to append "Don't answer if you don't know" to their question.

Why do people publish content-free answers instead of ignoring the question? What could be done to reduce this behavior?

(Don't answer if you don't know.)