The last time Americans could walk into a Las Vegas sports book during the Olympics and make a bet on their country, Michael Johnson was still winning medals, Vince Carter was making a towering Frenchman famous, and Michael Phelps was debuting in the Games as a 15-year-old also-ran on the U.S Olympic swim team. This year, all that changes. For the first time since the 2000 Sydney Games, Las Vegas sports books are accepting bets on the Olympics this summer, providing casinos a financial boost in the dog days of August, keeping bettors glued to the TV as Carmelo, KD,…