Often when I meet a potential client for the first time the very first question they ask me is, "Will you sign my NDA?" or "Will you bring an NDA to our meeting so that we can both sign it?" Unless the client has already generated a fair amount of intellectual property through development I will not sign an NDA. And even if they have developed something I won't sign an NDA at the first meeting. I am not in business to steal ideas, I am in it to develop theirs.
The first time I meet a potential client and that client meets me, it should be a fact finding mission where both parties ask the same question: "Do we want to work with each other?"
When going to a doctor or lawyer I don't ask them to sign an NDA because there is already an explicit agreement to confidentiality: they are bound by ethical rules of their professions; and there is an implicit agreement to confidentiality, that of trust: A doctor who snitched on you wouldn't be in business long.
Are we product developers that unethical that we harvest ideas like spammers harvest email addresses? Ours is an implicit confidentiality agreement: A business cannot compete with their clients. For example, a manufacturing company either sells to intermedate suppliers who sell to the public; or they sell directly to the public. They cannot do both and hope to survive. If our company went around stealing ideas from clients to compete with them, then we would have lost our clients' trust and would no longer be in the product development consulting business.
Thinking that developers and entrepreneurs are sitting around waiting for a good idea to come along is nothing but delusional. Almost anyone can have a good idea. What many misunderstand is that the value is not in an idea but rather in the implementation. I would estimate that the work required to bring an idea to fruition (and by that I mean, making money) is 99% of the effort.
The person who champions an idea is the most ardent supporter of it. They are the ones who wake up in the middle of the night to make notes and strategise about how they are going to implement it. It is highly unlikely that someone else will get that worked up about a mere idea, certainly not enough to pull developers off of another project to begin a new, unproven one.
Product development houses just don't have spare people sitting around waiting for ideas to come alone. We try and run a tight ship where everyone is busy on a project. And the other projects are more than likely further along the development path than a new, greenfield pontification.
So it's not so much about ethics, although we take it very seriously, but rather simple economics which enforces confidentiality and non-disclosure.