You don't actually understand what the presumption of innocence means, do you?

It doesn't mean that a person is presumed innocent at every stage of legal proceedings - otherwise, no-one could ever be arrested for anything:

"Well, you're standing over your wife's body with a knife in your hand that exactly matches the 23 stab wounds in her torso, but I must presume your innocent so I'll let you go."

The presumption of innocence means that when the case comes to court, the defendant does not have to prove that he or she didn't commit the crime - the prosecution must show that they did, or at least very likely that it is likely they did.