Presumably, the regulation is keep the cabin crew in control of alcohol consumption. They don't always succeed.
Consider the case of the late Gerard Buckley Finneran, a banker expert in Latin American debt instruments, who got thirsty on a 1995 flight from Buenos Aires to New York. You can read the details on TSG (page 2 cites the relvant regulation). Finneran was a member of the Air Force Academy's first graduating class.
Since he died of Alz 10 years later, in retrospect, that may have been the first warning sign.
Or he just may have been a mean drunk.