A few points. First, I like NASCAR and MLB. I used to like NASCAR a lot more, but that's beside the point. Again, it's all lumped into "entertainment," so whether people choose between NASCAR and MLB or MLB and NFL, it's still choosing. People make a choice how to spend that money.
For most people, the entertainment budget is a fixed amount. Thus, if ordinarily a person goes to X football games a year and Y baseball games, an increase in attendance of football games will require a decrease in attendance at baseball games--simply because there isn't enough money to go around.
Second, I don't see why you think DirecTV isn't a mainstream product. Where I used to live, in a rural area, everyone had a Dish. No one had cable. Where my old man lives, in a suburban area, it's pretty evenly split, judging by the number of dishes that I see attached to the sides of houses. I now live in an urban area, and I see fewer dishes--but to say that dishes aren't mainstream I just don't think is correct. Cable is certainly afraid of the dish, and I think the fact that the FTC (wrongly) blocked the DirecTV and Dish Network merger a few years back due to antitrust concerns demonstrates that dishes have significant market power.
Third, whether you understand why MLB is making this decision just isn't relevant. Again, I suspect that it feels that DirecTV will promote MLB more heavily, and given DirecTV marketing of the Sunday Ticket (how many commericals have you seen for Sunday Ticket? How many for Extra Innings?--there might be something to the theory of promoting interbrand competition), but even if this is wrong, it's MLB's perogative to market its product however it pleases.
Kerry's anti-capitalist stance is simply big government nanny-stating. He's a punk, and a statist.
I don't like Kerry. Just because I think he's an anti-American sack of crap, however, doesn't mean he occasionally stumbles upon something I agree with.
If you ask me, though, buying out your competition is the anti-capitalist move of the two. I advocate competition, not an artificially limited supply market with artificially limited demand.