[Goose]

QB slide rule is silly and sets up these sorts of hits. The Trojan was a classless jerk after the hit, probably could have adjusted to avoid it, but if QBs don't try this slide they will be much less likely to take a hit to the head, and they can initiate that slide any time they want. 

Maybe in the era where most QBs were pocket passers and few ran often, this sort of rule made sense to protect the klutz QBs.

Now, so many QBs are strong athletes carrying the ball on designed runs. If WRs and RBs can't slide anytime they like to avoid a hit, why should QB's get to do so?

ANY ballcarrier, including WRs and RBs (TEs too) can slide feet-first and end the play. The ball becomes dead and the play ends when any part of their body besides feet or hands touches the ground. Of course, QBs are so valuable to the team that in many cases they are instructed to slide and not take a big hit under "ordinary" conditions. OTOH QBs often take a big hit attempting to get critical first downs/TDs. 
 
It must be noted that unnecessary roughness can be called on plays where the runner did not "give themselves up". Similarly, targeting can be called on plays that do not involve a slide. The USC player was called for unnecessary roughness and targeting both, at least as far as I can tell. Targeting for certain. Unnecessary roughness is always going to be a judgement call. In some cases I disagree with the official's judgement. This case was not one of them. It was an egregious violation of both the letter and the spirit of the rules. One of the purposes of the feet-first slide is allowing the defensive player to avoid hitting the runner's head and upper body. The feet-first slide should make it LESS likely there would be helmet to helmet contact, not more likely. The USC player had plenty of time to avoid such contact by even a slight change of direction. He instead initiated an obviously avoidable forearm to the head of the QB. That was in addition to the helmet-to-helmet contact. This was in no way a corner case.

[Goose]

QB slide rule is silly and sets up these sorts of hits. The Trojan was a classless jerk after the hit, probably could have adjusted to avoid it, but if QBs don't try this slide they will be much less likely to take a hit to the head, and they can initiate that slide any time they want. 

Maybe in the era where most QBs were pocket passers and few ran often, this sort of rule made sense to protect the klutz QBs.

Now, so many QBs are strong athletes carrying the ball on designed runs. If WRs and RBs can't slide anytime they like to avoid a hit, why should QB's get to do so?

ANY ballcarrier, including WRs and RBs (TEs too) can slide feet-first and end the play. The ball becomes dead and the play ends when any part of their body besides feet or hands touches the ground. Of course, QBs are so valuable to the team that in many cases they are instructed to slide and not take a big hit under "ordinary" conditions. OTOH QBs often take a big hit attempting to get critical first downs/TDs. 
 
It must be noted that unnecessary roughness can be called on plays where the runner did not "give themselves up". Similarly, targeting can be called on plays that do not involve a slide. The USC player was called for unnecessary roughness and targeting both, at least as far as I can tell. Targeting for certain. Unnecessary roughness is always going to be a judgement call. In some cases I disagree with the official's judgement. This case was not one of them. It was an egregious violation of both the letter and the spirit of the rules. One of the purposes of the feet-first slide is allowing the defensive player to avoid hitting the runner's head and upper body. The feet-first slide should make it LESS likely there would be helmet to helmet contact, not more likely. The USC player had plenty of time to avoid such contact by even a slight change of direction. He instead initiated an obviously avoidable forearm to the head of the QB. That was in addition to the helmet-to-helmet contact. This was in no way a corner case.