You're probably right in principle, but for developers at a conference where the discs are handed out, or even developers who pay for one of the developer programs and get the discs that way, neither sign anything. Why you're essentially right is because there are those license things you agree to when downloading or installing software. It wouldn't surprise me if the small print in those covered the issue, and you are agreeing not to talk about the software with people. Literally though no signing of and NDA is involved.