Why is it a problem?
Honest question. In the mediterranean basin it was a necessity to collect rain water and store it in a big underground cistern.
Most people don’t do it now a days, but old houses still have the system in place.
If you use it, you keep the first rain waters of the season diverted to allow for roof cleaning, then you set the pipes to store the water during all the winter.
You also have to keep some control of water quality. The traditional method is throwing a lime stone in to the cistern to kill the patogens.
The western US has a different concept of water rights because there’s literally not enough water for everyone to do what they want with it. All the water that falls is already accounted for, possibly by someone way downstream of you. Even if you don’t live near a river, that water is draining to somewhere and spoken for. If everyone collects it then whoever has the claim to that water is being denied their property rights, most of which were allocated on the basic concept of “finders keepers” a couple hundred years ago during the settling of the American west.
The challenge is that people invest their money and lives (building businesses) on the fact that they have these water rights so it’s hard to change without creating high emotions on both sides (“water is a human right” vs the primacy of property rights)
If you take a few gallons, it's fine. If you and 10,000 other people take a few gallons, that a problem. So we need to regulate it a little.