The laws usually used to prevent rain catching are old ones intended to prevent people from doing things like redirect creeks and streams which feed other property. At one point in the wild wild west expansion of the beef industry, an unscrupulous person would buy land upstream of a rancher, stop the stream, and then use that to make money off the rancher. Today, it has been taken to the other extreme of making money. Now government sells water and waste removal and wants to force you to use their service.
In particular the desire to make you buy water for your garden or other use where they do not have to treat it later as waste because you are typically still charged for those services. So forcing you to use their water on your garden is profitable.