Yeah, that's a lame rule that the local broadcasting companies have imposed. It's the same reason that NFL games being broadcast locally are blacked out on Sunday Ticket - local channels want your eyeballs to watch their advertising. It's silly and has caused some big problems. For example, a couple of years ago in the NHL playoffs (remember that league?), the Wild/Ducks game 4 was supposed to be broadcast on ESPN. However, DirecTV was told that it'd be on a local channel as well (as had been the cast in the past), so it was blacked out on ESPN for the first period until they realized the local channel backed out. Everyone ni Minnesota missed the entire first period (and the only goal the Wild scored in the entire series ; they did give people who complained a $50 credit though, which paid for 50% of the entire NHL Center Ice package).
Along the same lines, people with the Ticket have been screwed when a game being shown locally turns into a blowout and the network switches to a different game -- meaning you can't get it locally OR on the Ticket (unless they're quick enough to switch it back to the Ticket channels, but that doesn't happen frequently).
Stupid greedy networks
CW