Q. Who cares if they know what I buy at the grocery store?
I could say the same thing about tapping your phone -- "Who cares if they know what you say to your friends?" How about putting a tracking device in your car? "Who cares if they know where you go?" Heck, why not let them install a camera in your shower? "Who cares if they know what you look like naked?"
The point is that there are many, many things that nobody's got any business knowing about anybody else. That's called privacy -- the right to an unfettered, unmonitored personal life which is not subject to the scrutiny of others. Hang on to your self-respect and don't allow people to nose around in your private affairs.