I'll get yelled at for this I'm sure, but I've always thought that the Aruban investigators got unfairly labeled. I don't know what else they were expected to do that they did not do. What they didn't have was even a scrap of evidence to link him to the crime. They didn't have a body. They can't prosecute without some sort of evidence. Sure-- he probably killed her, but at the time there just wasn't anything to go on.
I guess you don’t know that the Police Chief at the time was Joran’s godfather and the best friend of Joran’s father.