"Nope, we can't sell you that..."
Before long I expect to see similar "laws" forbidding grocery stores from selling food to fat people, or prohibiting restaurants from serving "obviously obese" customers.
Because, really, what's the difference?
In each case it's obvious that the customer has had (someone else's definition of) "too much" of what he is seeking to buy more of. It's also obvious that it's no one else's business- until a credible threat is made of causing imminent harm. And "might hurt someone" is never good enough an excuse for me.