As per 14 CFR 121.575(b)(1) (
https://www.law.cornell.edu/cfr/text/14/121.575):
"No certificate holder may serve any alcoholic beverage to any person aboard any of its aircraft who ... Appears to be intoxicated"
It is potentially a big deal if a flight attendant allows any passenger to become drunk, so I'm not at all surprised that they comment from time-to-time. In fact, I suspect they keep a rough mental tally of what they've served to every passenger.