Tracking mail for delivery and tracking mail regardless if it has tracking services, building a giant database, and handing it off to 3rd parties is different than signing up for a requested service. Information is already leaking out that the prism system is being used to collect information to sell to PRIVATE INDUSTRY to the highest bidder. In addition people don't realize how detailed of a profile of somone's life one can make with such little bits of information, and how easily that information can be used to hurt you regardless of the legality of your activities.
Source?
Just like before this whole PRISM scandal "broke" in the mainstream press, anyone who bothered to look could have found this information. Perhaps you should go look for yourself instead of expecting people to spoon feed you information which you will immediately decry and deny its relevance because it wasn't on CNN. I know the intelligence community loves to use its own mass media tools to expose itself and make its own job harder. Makes sense right? Go do your homework instead of asking some one else to do it for you. I don't give a shit if you believe me. This is laziness disguised as skepticism.