There are actually no laws on the law books that say collecting rain water in barrels simply from your roof or the sky is illegal.
So you can install a rain barrel and collect all the rain water you want too from your roof and gutters or from the sky directly and you do not need a permit.
However what can be illegal is stopping of rivers, creeks ponds etc and preventing the natural flow of rain water.
Harvesting rain water is perfectly legal in most states even though it seems like it's illegal due to the media.
What is illegal is damming up creeks, rivers etc and preventing the proper flow of water naturally to other places.
But simply collecting some rainwater from they sky in barrels or from your own pond is not illegal and you do not need a permit to collect rainwater.
When people get into trouble for rainwater the media doesn't report the entire details on what they were actually doing that made it illegal enough for them to get in jail.
They didn't go to jail for simply collecting rain water from their gutters or just out of the sky or there pond but instead the person who got into trouble for collecting rain water was blocking off creeks, rivers etc and re routing natural water flow and prevented it from getting to other lakes, ponds rivers etc.