All of the pontificating aside..The answer is quite simple to your query. Locals know a "Haul your rass cup" is cool as long as you don't make a scene. Tourists can walk the beach with a beer any time, as long as they are also cool and are not disruptive or rude. The law was made in the first place to keep the Island relaxed for tourists, as a few locals occationally abused the freedom to drink in public.