For what it's worth, the cost of watching baseball games (I've been subscribing online for two years) is a completely separate expense. "Entertainment" is far too broad and too subjective.
That said, what you're saying seems to be both for and against your viewpoint. If people have to choose between MLB and NASCAR (I personally can't imagine this scenario) this move would make the decision for them in many cases. I don't understand the move for many reasons:
1) DirecTV is a niche market, and even the extremely popular NFL has not caused DirecTV to become a mainstream commodity. MLB exclusivity would add something, of course, but I cannot fathom it would more than offset the loss of the multitude of cable subscribers.
2) MLB does not reach more people this way. They reach significantly less. DirecTV already offers the baseball package. I don't see the point of this.
3) I would imagine that MLB would consider this for the lump-sum payment DirecTV would have to give them for this exclusivity. Thus they would both encourage more people to sign up for MLB.TV and get a hefty bonus for this. (MLB wins because DirecTV would probably overpay considerably for this, as they do with the NFL TV package).
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.