Ok, then back to the wedding reception example. Imagine someone who is not an alcoholic, just a regular person who has a drink now and again. This person, if given access to an open bar, might have a few too many drinks and cause a scene, but if it was a paid bar would only have one or two and be fine. The person's behavior could therefore be 'controlled' by changing the availability to drinks; i.e. putting a cost on each drink reduces how many they have and the corresponding bad behavior they could potentially engage in if they had too many.