It seems that both of them should be held accountable. The drunk for getting drunk and the proprietor for getting him drunk AND facilitating a risky behavior (gambling). That is why you need a "cooling off" period or removing the liquor from the equation to reduce the risk.
We have a cooling off period for products. Why not gambling?
I see your point, but most gamblers (I have a relative who is one) know exactly what they are doing. They know they are going to get drunk. They know they are going to lose money. Therefore, they should have to accept the consequences.