You did more stating the rule than giving the logic for the rule, and the logic for the rule was the question. If you think there is good logic for the rule then spell out that logic. If there is some really good logic for it it hasn't occurred to me yet nor has anybody given it yet.
Who cares if a red goes on a pack breakout as long as you made the shot you were going for? Just spot it. You and I both know that when breaking up balls, that exactly where they all go is in many cases almost entirely luck and chance and completely beyond the players control. By having this seemingly silly rule you are allowing luck and chance to play a role they don't need to play as it did in the Ronnie breakout shot I posted. It would have been better to just spot the red and Ronnie continues shooting. He made his shot (the blue) and there is no need to be penalized for luck/chance that he can't control and that doesn't matter anyway. Again, so what that a red went, who cares, just spot it and player continues shooting.
Breaking up the reds is a massive part of snooker. I want to see what happens in a case like the Ronnie video I posted earlier be controlled by skill and only skill and so the only thing that should matter on that shot is whether or not he made the blue, not some superfluous luck stuff that didn't matter. With the current rules the pure chance/luck of whether or not one of the pack of reds you were breaking up after making the blue ends up going into a pocket or not is what is controlling whether you shoot again or not. Let me say that again, pure luck and chance are controlling who gets to shoot the next shot rather than skill.
The skill of making the blue should be what determines who gets to shoot the next shot, not whether or not he got lucky or unlucky about a broken out red also finding a pocket. Snooker should be above such silliness.