In most of the norteastern United States and adjacent Canada, there is abundant precipitation, and plenty of water. In fact, the Great Lakes alone contain about 84 percent of North America's fresh water. Most rainwater management consists of moving the water away from buildings and roads so it doesn't cause problems. But what about places where water is scarce? Do you own the water that falls on your home?
In the science fiction book Dune, the Fremen people live on a desert planet so dry that even the water within their own bodies is owned by the tribe and returned after a person dies. Water rights laws that strict do not exist on Earth, but in some places, until recently, the rain and snow that fell on you, your home, and your property did not belong to you! (this may actually still be the case in some places) In Utah, people were only given the right to collect rainfall this year, and only after registering their intent to do so. In Colorado, rainwater collecting was illegal until last year. I drove over Cottonwood Pass in Colorado last year and took the picture above of a summer thunderstorm. At that time, that rain JUST became legal to collect. I'm not sure if any got in my mouth when I drove through the storm and stuck my hand out the window.
In the West, people own rights to water in a creek or river, and it is illegal to take water from these sources if you don't 'own' it. It makes sense that people might be concerned that if everyone collected rainwater from their roof, it might somehow affect water supply. In truth, a study in Colorado found that the effect is insignificant (though I could not find a link to this study), and most collected rain water is used for irrigation anyway - most of it gets used by plants just like rain does, but some makes it into the water table.
Most of the West's water comes from snow falling in the high mountains, anyway. Not many people are up there collecting rainwater, and during snowmelt season you shouldn't have a rain barrel hooked up anyway - the freeze-thaw cycles will rip it apart.
So go ahead, harvest this water!