Actually, it is mostly because there is a rancher down stream that "owns" the water in the stream and needs it for his cattle. It is called "water rights" and when you collect rainwater, you "divert if from the stream" and you are stealing his water. Many a range war happened about water rights.
For instance, the US actually pumps water from deep wells into the Rio Grande in certain areas to make sure that the water volume in the river meets the treaty requirements we have with Mexico.
I saw an article several years ago about the "downstream effects" of the water rights war. It was about the Snake River and showed various pictures through history. The Snake River used to wind its way through the US and Mexico and empty into the Pacific. The pictures showed how the areas in Mexico became a desert wasteland because of cities and ranches in the US who are diverting/damming up the water further north. Of course, this was long before people understood environmental and economic consequences of those actions. I know water rights is a tricky and complicated subject but I still don't think it is right to say someone "owns" the water that falls from the sky.