NBC’s prime-time emphasis makes sense. People work during the day. Families still do most of their TV viewing in prime time.
But it’s growing clearer that people want to make that choice for themselves, not have it forced upon them:
Gallup: People want Olympics televised live and in prime time | Poynter..
Now here’s the flaw in NBC not wanting to “spoil” an event such as, say, their prime-time BMX segment by broadcasting the event live in its entirety:
To be truly surprised by what “happens” in prime time, people have to avoid news sites, sports sites, Twitter and Facebook. And yet NBC thinks people are going to go to all that effort but accidentally flip the channel to NBC Sports Network at 10 a.m.?
Duresport.com 
The problem is the potential for time-shifting, I think. The idea is that, if you show it on NBCSN at 10am, people can DVR it and watch it when they come home, then skip the primetime broadcast.
That’s why NBC is fine with showing events like this live online (which can’t be time-shifted) but not live on TV.
Not saying I agree with this strategy – far from it – but I think that’s the reasoning behind it.