That's just it, though -- people who are hopeless out-of-control misbehaving alchies will be so whether or not MK serves booze. They will bring their own, or they will get toasted at the bar of a monorail hotel.
So not allowing in the park doesn't deter them... but it does stop people who enjoy it responsibly from ever having a chance.
Also, look at it another way: What is more harmful, a moderate booze habit (which research has linked to heart health, longevity and a lower risk of degenerative disease) or smoking?
Yet smoking is allowed in designated areas throughout the park. Passing through or near a smoking area can definitely bring some guests back to the real world. Someone having a glass of beer or wine in a restaurant won't do anything to shatter that illusion.