There's nothing "flawed" about a policy that may encourage people to do something. A policy, regulation, law, etc., doesn't need to be bullet-proof to be effective. Are speed limits "flawed" because people are often able to exceed them without getting caught? Speed limits encourage safer driving. Are tax codes "flawed" because enforcement is difficult, there are loopholes, fairness may be questionable, and so on? Yet tax codes are effective in doing the overall job of bringing in revenue. This modest policy was limited to bottled water sales in the parks--and it may have been reasonably effective in reducing plastic waste because people drink a lot of bottled water, sales of which have been outpacing sales of soda for years. Beyond water bottles, though, I suppose the belief was that if a visitor realized the intent of the policy, he might think twice before buying a soda in a plastic bottle or having his groceries packed in a plastic bag.