In reply to Kibbitzer: You've convinced me to give your post another read.
Best I can tell, you are saying that presumption of innocence (PoI) does not kick in until an actual trial.
Next, however, you add another function to PoI which is to prevent "random" arrests, that is, arrests where the arrester cannot demonstrate probable cause. There is no point of making an arrest if the judge, as per habeas, will order the arrestee released the next day.
So, PoI requires (1) probable cause in the initial arrest and (2) a high standard of proof in a subsequent trial.
Now we need to switch hats from lawyer to political philosopher.
Why presume people innocent? That is the puzzle I am trying to solve. My answer is that in a free society, the scope of permitted individual actions is vast, and the prohibited things are few. If this were otherwise, arresting people randomly would net a huge number of actually guilty individuals and only a small number of innocents, making such arrests an exceedingly efficient from the social point of view practice.
Thus, sometimes libertarians say that these days there are so many minute rules and regulations of every aspect of life that everyone is perpetually guilty of something. If all these rules were just or efficient or both, then perhaps I'd be forced to advocate an end to the presumption is innocence. Such a presumption would impede justice.
For government agents the situation is reversed. Hence, the different approach to allegations of official misconduct that I suggest.
Look, a cop is essentially a robot, an automaton, allowed to behave according to a limited and precise set of laws.
It is for that reason that government work is basically unnatural to a human being. Only a special kind of person can be a police officer.
That is the essence of the idea of "rule of law": the law is made by the legislature (if positive) or judges (if natural) and is addressed to the police, strictly governing their behavior.
Rule of law is about subordination of the executive branch of government to the other two branches and how such subordination is to be achieved.
Our robot-cop is always and forever -- very properly -- under suspicion that he is indulging in something that is not compulsory and that therefore is forbidden. If such suspicion becomes too serious, as I expect would happen frequently in a free society, then the cop must be relieved from duty and thoroughly investigated. There would be no presumption of innocence for him for reasons already stated.
As a result, (1) the probable cause for relief from duty would be a simple accusation of wrongdoing. And (2) a trial under a tribunal would be much harsher on a cop than a normal trial is presently on a private citizen.
These are yet another part of what I call the "sorrow of authority." They're the price of power.