Miller has a lock on all beer at Nationals Park.
Mathguy, my Anheiser Busch source confirmed this is correct. According to her, Miller has 75% of the beer concession in the park, which gives them marketing rights they can use to preclude a lot of competition. Anheiser Busch is a major sponsor of the Olympics this year, which limited their ability to muscle in on the new Park. Miller effectively blocked Cap Cities by prohibiting signage aside from on the taps, and Cap Cities wouldn't bend that far.