You've missed an important point; while the data itself is already known- or at least available- to them, the sources that *you* choose to take an interest in say something about *you*.
No, I didn't miss that. I read Hut Six by Gordon Welchman well before the BBC made a documentary about him. I know how dangerous meta data is, so I always make sure I seed data that bears no relevance to my work. We pretty much pioneered the "poisoning the well" approach :).
This, by the way, points out something very important: I don't consider myself especially talented or clever. If I can work this out, there is no valid reason to assume evil terrorists are any less clever, ergo these measures are utterly pointless - at least if we believe they are there to catch terrorists.
Either the intention is to only catch dumb terrorists, or it's simply an excuse for legalised industrial and economic espionage on an unprecedented scale.
It's probably both..