That's just silly and bears no relation to the story at hand.
Or... based on the evidence, I can't prove you didn't do the crime. So I'll indite.
It's not up to the police to *prove* someone didn't commit a crime but it is up to the police to prove that they did when accused of a crime. These police were convinced by the evidence and testimony presented, that a crime was committed.
Three eleven-year-old girls went to the police and claimed they were attacked and showed evidence of having been attacked (cuts, scrapes, bruises). The above story does not provide all the details (more was provided in TV coverage). The girls picked out the "perp" in a photo lineup. The police erred (and have admitted to that err) in showing the three girls the photo line-up at the same time. Said "perp" was arrested. It's their word against a homeless man. So... "based upon what we had, we had no reason to think it wasn't true" is not out of line. The police did not suspect the girls were lying. No one did. That three little girls would collude and lie about such a story is what makes it so shocking.