Innocent until proven guilty doesn't mean someone gets to go free until their court date. It depends on the crime, the flight risk, and the supporting evidence that police are able to collect.

There are many examples of police letting suspects go due to lack of evidence and then later discovering they let the wrong person go. These stories generate a lot of outrage in cases where there's public interest or a news story, but this is the reality of crime: You don't always have enough evidence to justify detaining someone, but the police's job is to quickly try to find enough evidence to find the right perpetrator