The law as it stands in Colorado, for private boaters 'river running', is one coast guard approved pfd on board per person, and everyone under 13 years old has to wear an appropriate pfd. Many sections of river, regulated by various agencies, do require you to wear a pfd at all times. Utah stipulates flat water rules where wearing the pfd is not mandatory.
The thing about wearing a pfd in turbulent water is that it is just common sense. However most people don't have enough experience in this environment to have developed this common sense. This is a valid argument for making a law that mandates nothing more than common sense. It may be unenforcable, but potentially will save lives (big point here), as well as rescue efforts, time, resources, and taxpayer money to support rescue efforts (anpther significant point).
I guess my personal opinion is that it is difficult to make a blanket decision for all rivers, even in a particular area. But I am fully supportive of the individual agencies who regulate the various sections of whitewater rivers to be empowered to create a minimum mandate for common sense, not unlike a seatbelt law, for areas where this is appropriate.