One drink at a time and only served to seated passengers is a common airline policy and has been so officially for a long time. For example, KLM had this policy stated in their on board magazine a decade or two ago.

OTOH it hasn't been consistencyt enforced. The few times I've asked for hard alcohol on flights, it's been more likely than not that the FA handed two of the miniatures to me automatically.

Perhaps part of the purpose of the rule is to give amunition to FAs when they're concerned that someone's drinking could become a problem. I also appreciate the safety issue in serving someone who is standing or walking around the cabin. If I'm nearby, I would prefer not having some stranger's drink spilled all over me or needing to avoid broken glass on the floor.