Why this is a separate lesson
The previous lesson was a skill. This one is law, and law is the reason we split them.
Refusing service is not only a judgement call about a guest. In many places it sits inside a legal structure that decides who can be sued, who can be fined, whose licence is at risk, and occasionally whether the person who poured the drink is personally exposed.
That structure is not the same in every state, and in some places it barely exists. Which is precisely why this lesson will not tell you what your rule is. It will tell you what kind of thing to look for, and then send you to the body that actually sets it.
The vocabulary
Dram shop liability is the general name for a claim in which a third party — someone injured by an intoxicated person, not the drinker — sues the establishment that served the alcohol for the resulting damages. The classic shape is a crash: a person is served, drives, hits someone, and the injured party sues the bar as well as the driver.