What's the difference between the big screen in your living room, the smaller screen on your laptop or the tiny one in your pocket? Size aside, there's quite a difference, media executives say. To many, it's a mystery why we can't easily watch online broadcasts of the TV signals that are transmitted by networks over the air, on cable or via satellite. Even Bryan Perez, the senior vice president for NBA Digital, admits, "It's a challenge to explain." The simplified answer is that various media companies have a financial stake in where and how an episode of "Mad Men" can...