The legal issue of the ownership of rain water in Colorado is probably linked to the problems of the Colorado river/Lake Mead system. That barrel of water is one less barrel of water used to supply the residents of Las Vegas down river for example. Lake Mead has about 10-20 years left at the current drought conditions before it runs dry and when it does run dry 10s millions folks will become environmental refugees.

http://en.wikipedia.org/wiki/Colorado_River