Water law in the vast majority of the west is that way and has been for over a century. You can't collect and store rainwater. It is not because it belongs to the government. It is because it belongs to someone down stream who has a previous water right. The government has been pushing people installing rain barrels to collect roof run off water for their flower beds, etc. That is technically illegal, so they are giving people water rights to that water. It is a way to make a program they have been pushing legal. Go try to get a water right to pump out of a creek or even a well and wait to see if they give you that right. It isn't going to happen.