That's not how the burden of proof worked. It was established case law that any collected rainwater without clear water rights was infringing.

It's like if you went into fight a speeding ticket by questioning quality of the radar gun. I mean, in an ideal world you should be allowed to, but it's not going to fly. Our common law style system has this overarching goal of internal consistency, and there's been tons of cases where the radar (or maybe some relative of the radar, but close enough as far as the court is concerned) has been legally found in the past to be trustworthy. It's also in the Court's best interest to have this line of reasoning because it saves the court from having to rehash the same arguments saving the court immeasurable amount of time. About the only way you can get out of that situation is new legislation.

The courts here had gotten into a similar state where it wasn't really willing to hear new evidence on the matter, and needed legislation to patchfix. Something something checks and balances.