jg and Hawkeye, thank you for your responses. Unfortunately this is still what I'm having trouble with.
Absent illegal touching, if we have a turnover situation with a foul by B either during their run or during A's run, I don't see a provision in the rules that would not result in 5 and 10 yard fouls being declined. I think that part is obvious.
Yes, you are correct, if there is no illegal touching, then we decline all 5 & 10 yard penalties by the non-scoring team by rule and the touchdown stands.
However, in the instance when there IS an illegal touching of a kick and there is a touchdown on the play, we must determine if the 5 or 10 yard penalty CAN be enforced. If it can, then give the option to the scoring team. If it can't, then ask about the illegal touching.
The reason that the penalty that occurs during a touchdown run (the actual run that scores) by the non-scoring team can't be enforced is because the enforcement spot is the goal line and enforcement would take you into the end zone. If, however, the foul occurs during a non-scoring run, then there is an enforcement spot (either the end of the related run or the spot of the foul).
I think you may be getting stuck on the fact that there is also a spot for enforcement if there is a foul during a non-scoring run when there is no illegal touching. Yes, there is, but that scenario doesn't fall under the criteria for 10-2-5-a-2 (i.e. no illegal touching), so we decline by rule.
It has nothing to do with the illegal touching per se, it is just that we go looking to see if the penalty is enforceable IF and only IF there happens to be an illegal touching during the play.
The reason this came about was because team A wasn't going to get a touchdown on play because they committed illegal touching and the foul by team B was only 5 or 10 yards. So the argument became: well why can't we accept the penalty if it will result in team B starting out further back than the illegal touching spot, after all they fouled too.
As for the exception in 5-2-4, I believe that it should say "Exception 10-2-5-a-1". In the 2008 rule book, the exception read "Exception 10-2-2-g-1-a" and 10-2-2-g-1-a became 10-2-5-a-1 in 2009, whereas 10-2-2-g-1-b became 10-2-5-a-2.