IMHO, Sagan knew what he was doing, horner is right in that case that he cant break, the gap was there and the jury is right to relegate Sagan, because it's dangerous. But all the sprinters know it's dangerous. In the end, the decision to go for it or not is on the sprinter. In such a case, as the UCI rule is quite clear, he has to count with the relegation if the gap is closed.
However, Sagan cannot be blamed for the decision to go for it. Yes it's dangerous and yes, recently there was pretty severe showcase of consequences if something happens. But all the sprinters accept the risk. Otherwise they wouldn't be there. You cannot get rid of it, because it would be like to tell the boxers just to do a shadow fight, because they could get a concussion, or worse, a brain damage. It's a sport for christ sake, and many of the best and most successful sportsmen and sportswomen are arrogant as hell. And they need to be, unless it stops after the race.
Today, sagan again showed rising form, it may not be enough for the green on tour, but pissed(in a good way) Sagan in a good shape will make an impact in the Giro. Maybe even points jersey and multiple stage wins.
EDIT: It would be nice though, if the resolution of the green would happen only in Paris. Somewhere around 20 points difference would be the best.