For the legion of rich and famous in New York, the unofficial start of summer means migrating east by luxury vehicle to the Hamptons, that slice of exclusivity at the end of Long Island. But this Memorial Day weekend they were greeted with a jarring new sight along an otherwise piney, bucolic stretch of Sunrise Highway, the main artery leading to the Hamptons: two six-story illuminated billboards being hastily constructed by a local Native American tribe just in time for the high season. Tall enough to rise above much of the tree line of this state roadway, the twin billboards...