If all suspects are innocent until proven guilty, then isn't it wrong for police to hold a person in custody since they are innocent
It is not wrong to hold a person in custody if they qualify to be a suspect. This is because, to be a suspect, there has to be some kind of information or evidence that is pointing to you.
The term is "innocent until proven guilty in a COURT OF LAW.". But before the trial the accused may have to stay in jail.