They need to provide live streaming coverage of a wide variety of events, whether for free (with ads) or for a fee. That's the minimum bar these days, if they can't meet that bar people will just get their olympics coverage some other way (such as pirated bbc coverage) and NBC will still have lost out on a massive amount of potential revenue. And in the meantime they will have tarnished their brand as well.
By making this stuff available online, the amount of money they can get for the primetime TV spots is reduced, because fewer people will watch the primetime stuff, and it has less cachet because it's no longer exclusive. They will recoup some of that loss by charging money for internet access and having ads on the internet stream, but the ads online will be even cheaper, because that's just how internet ads work. Not only that, but they have to have a platform that they can use to deliver and charge for this content at scale (they might already have such a platform, but it may not be up to this kind of scale, I'm honestly not sure.) So that costs money, too. It's likely the folks at NBC did this analysis and found that it didn't work out in their favor, or there was too much uncertainty.
Meanwhile, most people will be at worst annoyed by the lack of internet coverage, but they'll just record it on their DVRs, or miss out on an event here or there. A very small number of people will go the trouble of pirating it, but those people are likely a margin of error in the grand scheme of things.
What's not clear, as with the newspaper industry, is what the alternative is.
Henry Blodget, tarnished prophet that he is, has been beating this drum loudly and fairly convincingly for a few years now:
http://articles.businessinsider.com/2009-06-12/tech/30062877...
But who am I kidding? Nobody's going to stop watching NBC permanently because they won't air the olympics in a timely manner.
An option could be to double down on the tape delay, and uniformly embargo results until the airing: pretend that when something airs on NBC is when it really happens, at least for NBC-property purposes.