Why is it like this? The reason is quite simple, in a D/s relation the Dom makes the decisions and the sub follows them. Any breaking of rules is therefore in the first place a departure of this principal that has been mutually agreed on. (the presumption here is SSC). The broken rule it self is therefore just an event that could almost be considered arbitrarily. Any other rule could have been broken and it also really does not matter which one it was. What only matters is that if left unpunished the principal of the D/s relation is in decline. And this has to be stopped and if needed reversed if it is desired that the D/s relation is to continue. Considering this it could be argued that a rule does not even has to be broken for a punishment to take place. At any time when a Dom is convinced his sub is departing from her submission and this is undermining the nature of their D/s relation he may decide to punish his sub.
Now comes the matter of finding an appropriate punishment and this should be carefully tuned to the character of the sub. Why should this be done? Why should a Dom change his ways to accommodate his sub? Also here there is a simple and pragmatic answer. His sub is, thank God, in the modern western society a free person. Not only does she not have to tolerate any inappropriate punishment, it is also in the interest of a Dom to keep the end goal in mind with everything he does with his sub. And this end goal is submission and obedience. This may not appeal to some Doms and Subs, but it is the reality.
So now that A Dom knows that he has one more tool in his tool box to manage his sub, he can start understanding how she thinks and feels about punishment. And he can adjust or compromise his reactions to get the best results.