"If I know the tweets and news and other texts you consume, and I cluster them, then I can very quickly determine your set of interests / sentiments / whatever clustering regime is applied,"
Yes, and that's the point. They're not allowed to gather info directly from you unless you grant them the privilege. However they no longer need to get direct access to you, they can work out who you are and what you like simply by piecing your life back together from the trail you leave behind. Why do think big corps like Google and Facebook aren't fussed about your granting them access to directly collect info about you? They simply have so much reach and so much data about you they know all about you already!