Was Romulan Ale illegal because of a trade embargo between two warring states, providing material support to the enemy ala Cuban cigars?
Or is it because the Federation nanny state thinks alcohol in general should be banned, ala Prohibition (at the same time making the black market sale of alcohol lucrative)?
Or is it because something specific about Romulan Ale that makes it illegal? Toxicity? After-effects? Promiscuity?