First of all, let's set aside any legal implications for now, as those are questions best answered by lawyers (and will likely vary across jurisdictions, etc.) and focus purely on the ethical.
With that in mind, let us suppose that person A creates and hosts an online Q&A community. Person B writes a question. Person C answers said question. Person D reads the question and answer, and develops and commercializes a product based on C's answer.
The question being asked here is: Does D's product constitute an ethical violation against person B (the asker of the question)?
In order to answer that question, let us consider: if it is an ethical violation, what ethical norm has been violated?
It certainly isn't theft (simpliciter) or even "theft of services" (read "theft of intellectual property") as there is no generally accepted ethical principle that would grant one exclusive rights to exploit the answers that are derived from a question one posed-- unless there is a contract stating otherwise.
In other words: the only ethical violation would be a violation of a commitment (implicit of explicit) one made while taking part in the community. In practical terms, this would be spelled out in the Terms of Service, but from an ethical perspective, the more germane part would be the prima facie assumption that one recognizes that anything one publishes (and writing on a publicly visible site surely constitutes publication) would put the ideas in said publication out into the world for all to see and use unless one has taken explicit prior acts to protect said rights.
Put another way: if someone has some valuable information, and then publishes that information in a freely available form, I don't see how one could claim that the recipients of said information have any ethical imperative to refrain from making use of that information for their own enrichment.
And in this case, the situation is even more clear cut, as the question relates not to the answerer of a question, but the asker. I don't see how anyone could expect there to be an ethical imperative constraining people from making use of valuable information that one did not directly create, but only indirectly inspired through the posing of a question.
Now, as I said, all this changes if there are explicit contractual obligations specified: but these would need to be accepted by the recipients of the information prior to receiving the information-- which means that we are not speaking about general publication any more.