This is an interesting problem and hinges on the difference between a current state of an individual and a predisposition. A bartender can be expected to identify obvious symptoms of drunkenness and withhold alcohol. On the other hand, a bartender could not be expected to diagnose an underlying predisposition to alcoholism. In other words, if an alcoholic who is perfectly sober at the moment, walks into a bar and orders a beer, the bartender is not expected to somehow divine the underlying medical condition and refuse the person the beer.
Similarly, there is no way that someone at the counter of a convenience store selling lotto tickets could reasonably be expected to diagnose gambling addiction in a customer who asks for a lotto ticket along with a soda and bag of pretzels, for example.