Not all of the deleterious effects of alcohol consumption are weight-related.
No but blood alchohol level is. As you say the health consequences of this (if someone is doing this often) is not AA's concern.
The main thing is, it's a lazy and patronizing policy. If someone appears intoxicated, whether they are disruptive or not, FAs already have the ability to refuse service.
If this is domestic only, 4 is not an onerous limit, although a few on the transcons may want more. How does the AA premium product stay competitive with this policy, if it is true?