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.