Here is a list of many states laws in regards to rain water collection...
http://www.arcsa.org/?page=273
Worth noting is that just because the law might be rarely enforced does not equate to violating the law being legal...
I find many laws on landscaping and gardening to be silly, for example laws that prohibit gardening in the 'front yard' or gardening altogether in some areas... I also find laws that mandate a 'lawn grass' height or forbid certain types of landscape to be disturbing, or worse home owners associations that mandate you not only maintain the height of your grass but that you also water your lawn and treat it with herbicides to kill the 'undesirable' greens in your lawn...
And I have been a victim of this, I had a neighbor that was beyond a busy body when I lived in a neighborhood, she would call the cops on me any chance she could... And although this may sound far fetched, or even mind boggling, I found out from another neighbor (stay at home) that he literally saw the cops out several times each month with a ruler measuring the height of my lawn my stay at home neighbor wanted to know what was up with that... After I explained about the busy body, said neighbor would start up their lawn mower and start mowing my lawn every time the cop showed up after that. much to his annoyance

Never got a ticket for that, but the idea that the police department was actually wasting time measuring the height of grass with a ruler suggesting that it's not in clear violation of the height law is disturbing to me... She also filed a report that my 'bushes' were too tall in my yard, and I actually got a frivolous ticket for that one... Let me say the conversation I had with the chief of police over his officer issuing said ticket wasn't friendly... I kept asking the chief of police to show me the law defining the maximum height of bushes and asked him if their was a maximum tree height as well, lets just say he got real flush in the face real quick and babbled a lot... He called back later day stating I was violating a law in regards to bushes/trees obstructing the view of an intersection or sidewalk, when I explained I didn't live at an intersection, there is no sidewalk in from of my house and that none of my bushes were even close to the road, the closest being about 30 feet into my yard, he said he would look into it further and get back to me again, I never heard back and never paid the ticket either... I also got a ticket for 'noxious weeds' in my yard once, as defined by my local ordinance they use a list of weeds defined by the state and that list only contains 9 plants, I had none of those defined weeds in my yard, so again an argument with the chief ensued over the ticket before he dismissed it...