Zwischen dem ersten und letzen für freie Fahrt validierten Tagen berechtigt der Swiss Flexi Pass zum Bezug von Billetten zum halben Preis.

The key is the meaning of zwischen, which translates as between but in German usage can (and here does) include the first and last, as in "from the beginning of the first day to the end of the last day" - it's one of those linguistic subtleties that make life interesting.

In any case, i'd buy a pass in the U.S. if you're an American and the language on RailEurope's sight will be what counts - ask the seller - Raileurope, BETS, ricksteves, etc to guarantee that the language will be honored by them if it isn't in Switzerland - i'm sure they will say they will - then if you get charged, which i'm positive you won't, save the receipts and have them pay. Buying passes in the U.S. also are currently cheaper than buying the same pass in Switzerland last time i checked and perhaps the wording that comes with the pass in Switzerland sales is not as clea.