They didn't arrest him for being guilty, then arrested him on suspicion of being a fugitive. They can do that and hold you for a prescribed period of time before they must charge or release you. I think it's 24 hours but that may not be true.
If they could only arrest guilty people, nobody would be arrested because it's not their job to determine who's guilty. The courts couldn't find anybody guilty because the police couldn't arrest anyone.
I imagine only innocent people would show up for trial, voluntarily.