I watched the replay from the angle you are talking about. First of all, your thread title saying Nebraska is "lucky" it wasn't called, at least from my point of view, implies it should have been called and wasn't. I don't see it. I see what you are saying, about how a ref looking for a make up call could have decided to use that, but he would have been on very thin ice. As bad as the B1G refs have been this season, and were in that game, this isn't our final season in the Big 12, and I'm fairly certain Greg Burks was nowhere near the field.
1st off, you have to realize you're watching that replay in slow motion. At normal speed, and you can even see it in the slow motion replay, you would see that when Collins starts to wind up to make the hit, Cook has just started his throwing motion, meaning there was no reason yet for him to slow up.
2nd, he really hits nearly instantly after his release. Again the slow motion kinda messes up the perspective here, but you can see he's still following through on the throw when the hit occurs.
3rd, the hit is relatively minor, little more than a light shove. If not for the pile of lineman in front of him, he probably takes one steadying step forward and nothing more. As it is, he kinda just lightly falls on the pile in front of him.
Also telling is the fact he doesn't appear to try and lobby the refs for a flag, and most any quarterback I've seen will cry until the cows come home to the ref if they think there is anything even remotely approaching a late or excessive hit.