The rules must take care of two things (I think Shamu put it so clear a few posts before):
a) Balance
b) Punishment for unsportsman behaviour
If I'm hit and I don't leave the field (if I'm aware or not is academical) I'm having an effect on the game. Be it that I'm still shooting, be it that someone is forced to continue shooting at me or even watching if I'm leaving or not. If a player that should not be having an impact in the game is doing so, balance must be restored.
If I'm hit and I try to wipe or I try to continue playing then I need to be punished for not showing a sportsmanlike conduct.
Using Nick's example, if I while trying to reach the soccer ball hit a guy in my goal area, and he falls thus missing an oportunity to score, the ref will call a penalty shot, trying to balance the situation (they had an opportunity, my acting made them lose it). If I punch him on the face, the ref will show me a red card and call a penalty, trying to balance the situation *and* punish my unsportsmanlike action.
There will always be grey areas, but they can't be near the substance of the game. A ball goes thru the hop or it doesn't, a runner steps on all four bases before the ball reaches any of them or he doesn't, a baller is eliminated or he isn't.