In Brad Lancaster's Rainwater Harvesting Basics YouTube video, he mentions that in Arizona though rainwater collection was illegal people collected it anyway, and the laws that made it illegal were changed. Perhaps you might think that rainwater catchment was illegal because of some concern or another about cisterns, but according to an article in the LA Times, collecting rainwater off of one's own roof in Colorado is illegal and considered to be stealing water that belongs to someone else.
Powerful lobbies oppose changes in Colorado water law that would allow rainwater catchment. They see the water that falls on a roof as already being part of a waterway whose flow has been allocated.
Water law generally is complicated and Texas water law is no exception, but the simple reason rainwater catchment was not a big legal issue in Texas is that though the state controls natural surface waterways, rainfall is not considered part of the waterway until it joins it. Landowners can reasonably capture and use rainfall before it joins a natural stream, creek, river or lake. Though we have plenty of issues for our legislature to fight about, thankfully rainwater catchment is not one of them.
So, legal rainwater harvesting is yet another reason to be glad to be a Texan.