Please, go back and read the post. The only reason he walks is because Batman conducted an illegal search to obtain the knife and broke the chain of evidence on it, rendering it inadmissable as evidence.
And that right there is glorious proof as to why Batman being any sort of valid law enforcer is ludicrous. Any evidence he provides is unsafe and untestable in a court of law because it's virtually impossible to disprove that he didn't simply beat someone up and plant evidence allegedly proving their guilt on them. And given the fact that in his own books Batman is frequently portrayed as an urban legend, on top of his continued violation of both criminal and civil law, it's not like he's able to benefit from the Mask Law that allows costumed crimefighters to testify.
The only conceivable way evidence he provides directly instead of tipping off the cops about is if there's some form of independent corroboration such as photographic or video footage from a third party.
If the sister had tipped the cops about her suspicions, a legal search would have found the murder weapon -- significantly, quite possibly before it had been used on its third victim -- and maintained the chain of evidence, removing the technicalities (arising from Batman being out of his emotional depth) that set Branneck at liberty.