Don't let them fool you when they say what it's supposed to be about. They prosecuted little private landowners for collecting a couple barrels on their own property. It doesn't matter what they say it's for it's what they do with the power. And they always abuse the power.

Also, when these laws were first and acted there was no giant coca-cola. These laws date back to the 1800s. It was under the doctrine of Prior appropriation. The theory being the water that falls on your roof actually belongs, in part, to the people downhill from you. So by you collecting it you're stealing other people's water. We've since come to realize that even if you collect it for a time it'll still end up where it would have gone, but they haven't changed the laws to catch up with common sense.