That's a good point but even a total surveillance system has limits of actionability. Any given response system or mechanism is going to have limits of attention that it can apply to actual and justifiably observed behaviour. That is, if the goal is to identify, and reduce, negative behaviours, then whatever it is that your policing mechanism is can only act on some n cases in any given period of time.

The problem is that reducing negative behaviours in a just fashion is not the only, or even the most likely, possible dynamic for such a system. In a tyrannical regime (political, social, economic: this is not strictly related to politics), there's a great deal of power and control to be had by acting arbitrarily, without predictability, and without recourse, at any given time.

Take the example given in Paths of Glory, a 1957 film telling the story of the (arbitrary) execution of three French soldiers who had refused to undertake suicidal attacks in the trench warfare of World War I. The process draws on the ancient tradition of decimation, which is the arbitrary killing of one in ten of ones own troops* in order to instill on the remainder the consequences of failure to follow command.

The real risk of global surveillance is that it provides either the justification for, or the plausibility of justification for, arbitrary prosecution of any given individual at any given time.

That's an absolutely despotic state.