Any revenue they could make with online ads would be irrelevant compared to the TV contracts. Here's some anecdotal data:
Baseball:
- Fox pays MLB $625 million per year to show the World Series, every other NLCS and ALCS, the All-Star game, and 26 Sunday Night games. In addition, Fox pays $43 million per year for an additional 2 games during the week with no exclusivity.
- Simultaneously, TBS pays for one Sunday afternoon game per week, and the remaining NLCS and ALCS games not shown on Fox. I couldn't find a price-tag, but based on similar deals $200MM/year is about right.
- On top of that, ESPN pays $80 million per year for 2 games on Wednesday and the Sunday night game.
- In addition, the rest of the games are sold to major and local networks. There are > 2400 MLB games in the regular season alone.
Football:
- ESPN pays $1.1 billion per year just to show the Monday Night Football game
- NBC pays $600 million per year for the Sunday night game. Interestingly, the Sunday night game is the most expensive 30 second spot on a recurring TV program, at $400,000 per spot.
- If you're at all familiar with the NFL, you'll know about the ongoing drama between the NFL network, Comcast, TWC, and DirecTV. I don't know what DirecTV is paying for the rights to broadcast every game, but I would be shocked if its less than $1 billion per year.
And that's just some of baseball and football. There's no way in hell online ads are worth threatening that cash cow.