re: Why is it illegal to catch rainwater??!

It's not illegal in the majority of states. In Louisiana, it is legal to collect rainwater. The only thing that is illegal is reusing it for interior uses, such as for toilets, and that's only because DHH hasn't moved into the 21st century. I have designed a good number of rainwater harvesting systems. Everything from 55 gal rain barrels to 40,000 cf vaults under pervious parking lots. Rain water harvesting is encouraged in almost all urban areas, outside of a few western states. It costs money to handle stormwater. The more you keep on site, the less that enters their system and contributes to downstream flooding. There are other incentives such as better water quality, aquifer recharge, aquifer draw down, and things of that nature. Say in New Orleans, the more we capture, the less we have to pump and the more we can contribute to stressed groundwater systems.

In places where it is illegal, it is primarily due to stresses on agricultural and drinking sources in arid environments. If everyone started harvesting rainwater upstream, there wouldn't be enough downstream. There are some good arguments on both sides. I'm not quite sure where I stand on it. I tend to thing harvesting on a small scale for personal use, maybe say a certain limit on volume or square footage of collection space, should be allowed in those areas where it's currently illegal, but I don't really know.