Just to Chime In, the Black Market is not particularly 'American'. Some sort of Unregulated Retail Trade has been a feature of societies going back at least as far as Classical Greece, so it's more than a stretch to try to confine it to an American Unique.
By the same token, the Speak Easy was a reaction to an attempt to prohibit a popular Vice in one time and place (Specifically, about a decade, or 2 to 10 turns In Game). More generally, limiting the time and place you can drink alcohol is, I think, historically more associated with England (the traditional 'Pub Hours') than America, where such restrictions were almost always in the hands of local jurisdictions rather than a general, Civ-wide phenomena.
Besides which, in general, events and decisions that even at the time resulted in almost wholly negative and unwanted consequences to the Civ, like Prohibition and/or general Law Breaking, are a poor choice for a Unique to represent that Civ. I suggest that culturally, the Jazz Club would be a better American representative Unique.
Another suggestion, for a Unique that to my knowledge, has never been contemplated: The Spreme Court. The idea of a separate legal check on the 'sovereign' authority of the government (either Monarchial or elected) is very nearly unique to the American system.