Why do the thought police allow Julia and Winston to carry on their relationship as long as they do?
Since the screen in their room was presumably there the whole time, and as Big Brother often silences people due to mere suspicion that they are free-thinking, it just doesn't make much sense to me why the thought police didn't detain Winston and Julia in the first place.
Sometimes, you have to give people enough rope in order that they will hang themselves with it.
In the instance of Winston and Julia, the affair was probably not a secret as they hoped, but the trap was set probably to catch Julia more than to catch Winston. He was just an innocent victim in the plot to entrap Julia.
From the beginning of the book, Winston appears to be compliant but somewhat dissatisfied with the whole social order. It is not until he meets Julia, falls in love with her and begins an affair that he realizes that he does not love Big Brother as a good party member ought to.
I believe one of the many possible answers lie in the fact that while the saying might be 'big brother is watching', he only in fact two eyes. We cannot expect everyone to be watched constantly, that would be foolish, wouldn't you say?
The longer Julia and Winston continued their relationship, the deeper their hatred and opposition towards the party grew. The party eventually destroys Winston and Julia's relationship, and makes Winston say he loves Big Brother. This shows how any opposition to the party, no matter how strong, could be changed.
well my good sir, i believe the thought police wanted to get as much evidence of his thought-crime as possible before taking him away.