gregwiggins and essxjay, I understand that this is the standard operating procedure for journalists. However, when speaking to someone who is not used to dealing with the media, it would seem to me to be an essentially costless courtesy for a reporter to ask "may I quote you on this?" as well as "may I talk to you about this?" It's costless unless the reporter thinks that people might clam up if they asked for permission to quote, which would be all the more reason to be clear.
I don't know what is or isn't common knowledge, but the fact that reporters have a convenient shorthand doesn't necessarily mean every understands it.