I find it strange that ESPN and TNT don't show ads when streaming live sports. Instead they display a static image that says, "We'll be back shortly." Intuitively, it seems like they're missing an opportunity for additional revenue.
My suspicion is that they're waiting until they have a large enough audience to justify high prices for these ad placements. Because streaming live sports is relatively new, their numbers are still much lower than broadcast and cable. As a result, advertisers would demand lower prices, which would be difficult to readjust when the audience eventually grows. So, instead of anchoring prices too low, they've opted to refrain from collecting revenue today in hopes that they'll be able to demand higher prices in the future.
While this makes sense, I'd rather they show something more entertaining or at least play some music.