That's always been the preferred defensive technique of defending the option - man coverage on all potential ball carriers and "tackle" them every play. Makes the qb pitch faster and the rb's take their focus off the pitch.
As officials, we know the "tackle' can only be a legal block on a non-ballcarrier. Now, with the defenseless player/out of the play emphasis, a decision has to be made about the QB after he pitches the ball. After all, it is football. You can't make the defender have to sit back and wait on the QB to come to him because of fear of penalty in the event he makes a last second pitch. At the same time, he can't be fair game either.
I think this is in need of an AR on the NCAA level and a casebook play in the NFHS.