Goldgato
It refers to an expectation of privacy that is generally recognized by society. Society recognizes that you have an expectation of privacy when you go into a bathroom stall, but not when you go out into a public place, for example. Or when you post on Gaia you can't have a reasonable expectation of privacy because it's a public space on the Internet. Generally anything in a public space that can be readily experienced with any of the five senses doesn't have a reasonable expectation of privacy, so what you do inside of your car might be protected but not an order coming from it.
Oh, I see. Are you suggesting then, that we as a society expect our interactions on free-to-join internet forums to be private and you disagree with the rationality of that?
I really don't know what American society thinks of internet privacy, but I would imagine the expectations would vary based on the individual and their awareness of website functionality. I'm sure there are plenty of GaiaOnline users who don't even realize the forum can be read when not signed in.