"No drinking at work events."
".. If you want to drink, be my guest. Drink. You're a grown up. I'm not the boss of you. But don't drink in a situation or event that is officially connected with work in any way."
I think this idea is due to the over consumption of alcohol that seems to be the norm in the U.S. now. Europeans, on the other hand, seem to have much more mature attitudes around alcohol.