When should the whistle blow the play dead

[NOWIS]

The play that Locke got hurt on brings up the question. I understood the ruling on the field when they picked up the flag for the personal foul penalty because the whistle had not blown. The question is, why hadn't it been blown? When the ball hits the ground on an incomplete fwd pass, the ball is dead and the play is over. Shouldn't the whistle be blown to protect players from just such things?

[lighthouse]

I just watched the replay several times, and there was a whistle as soon as it was obvious the pass was incomplete. The MSU player was already going for the loose ball when the whistle was blown and there was no way he could stop. There was no intent to injure Locke,and he injured his ankle not his knee. They were correct to pick up the flag.