i would guess that the flight attendants, most of the time, are too busy and overworked to bother with the monitoring of nips. they cannot be everywhere and overseeing everything.
san nic hit the keyword........discreetly
i am wondering just how many participate in this "crime" ?
Being discrete doesn't make it legal. The vast majority of in-flight incidents involve alcohol, something like 90% was the last figure I saw from the FAA when I was still in the training department. A lot of people don't realize that alcohol impacts them very differently at altitude. Each drink feels like 2 or 3 once it is in your system.