Let me ask you:
I assume that you craft a budget for your family's expenses. So when you craft your budget, do you have a category for "entertainment," or do you have a category for each individual entertainment expense: MLB, minor league baseball, NFL, NBA, Broadway shows, movies, DVDs, CDs, etc?
I'd be willing to bet that you, and the vast majority of people, budget according to one large category: entertainment. Whether or not you like NASCAR or basketball or hockey is precisely my point: MLB and NASCAR and the NBA and everything else are all competing for that little slice of pie in your budget that is labeled "entertainment." The fact that you choose MLB over NASCAR doesn't mean that other people don't choose NASCAR over MLB.
I think your perception of how people spend their money is incorrect. Maybe people break sports into their own compartments when talking about likes and dislikes, but I bet very few go so far as to compartmentalize them when it comes to their spending habits. In fact, I'd bet that the only people who would do such a thing are people who have season tickets--and even then, I bet they only break out the cost of the tickets themselves, with any ancillary expenses falling into the "entertainment" section.
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).