Yes, this is actually quite interesting. I have just written a paper with Ven. Analayo (not yet published) on the role of the origin stories in the Vinaya Piṭaka. My contention there is that the vast majority of origin stories fit perfectly with the rules they belong to. But the closer you look at this, the more you start to wonder. It seems that there are actually quite a few origin stories that are slightly at odds with the rules. So why is this the case, and what might the implications be?
The first point to make is that although the introductory narratives may not fit the rules perfectly, they are still closely related. Take bhikkhu pārājika 4. The rule is about falsely claiming supernormal powers for oneself, but the origin story is about falsely claiming that others have supernormal powers. The two are closely connected, but technically the rule does not follow from the origin story. This fact of a close relationship that doesn’t quite work is particularly interesting, and you find this with a number of rules: bhikkhu pārājika 2, bhikkhu pārājika 3, bhikkhu nissaggiya pācittiya 4, and probably others that I haven’t considered.
It has been suggested by scholars (I can’t remember who) that this slight mismatch between some origin stories and the rules is a sign of the lateness of these origin stories. I am not sure if this follows. If the origin stories were created artificially to match the rules, wouldn’t they have been created to fit the rules perfectly? That’s what I would do if I were in charge of such creation.
In fact, some of the origin stories - such as those for the sekhiya rules, the least important rules in the pātimokkha - do have a number of characteristics of being artificially created: the rules are almost identical in phrasing, it is always the group of six monks who are the perpetrators, the stories are very short and pro forma. And tellingly, the origin stories simply state that the monks in question just did what the rule prohibits. So the fit between origin story and rule is perfect.
So it seems that a perfect fit may in fact be a sign of lateness. Indeed, it’s what you would expect if someone had simply made up a story to fit a rule. But how then do we explain that earlier origin stories deviate slightly from the rules they comment on? Actually, the more I think about it, the more natural I think this is. It seems to me that the Buddha would have reacted to monks’ and nuns’ misbehaviour by laying down suitable rules, not necessarily rules that precisely reflected what the monks had done. The Buddha was concerned with safeguarding the Dhamma for the benefit of all, and he would have tailored the rules to this purpose. The Vinaya is not like a code of law, where the law is written in response to precise misdemeanours. The Vinaya is really an extension of the Dhamma and serves the purpose of supporting the liberation of human beings. We probably should not expect the origin stories to always fit the rules perfectly.
Another possible source for the discrepancy comes from the fact that the origin stories were added some time after the rules were laid down. (This is generally accepted.) I think it is likely that in many cases the Sangha would still have had a memory of the actual events that led to the laying down of a particular rule. But the memories being memories, they would have been distorted, especially since these stories would not have been handed down through the standard process of oral transmission. The combination of an unreliable memory and trying to faithfully record the original events may have led to origin stories that did not properly reflect what actually happened and thus did not properly fit the rules they belonged to. In sum, I believe the imperfection of an origin story may in fact be a sign of its authenticity.
So what then about bhikkhu nissaggiya pācittiya 15 (NP15)? This rule has a very interesting origin story, since so much of it, as you rightly point out, has very little to do with the rule. I think a lot of what we see in this story is an expansion of the Vinaya through the adding of new material. This is not unique to this orign story. A number of them contain sanghakammas (basic legal procedures of the Order) and other rules that are only found there. (Again, see the study I am about to publish with Ven. Analayo.) In other words, the origin stories contain unique Vinaya material which makes them an important source for our understanding of how to practice as monastics. Some of this material is not really relevant to the pātimokkha rules, but was probably included as a convenient way of making it part of the Vinaya.
This, I believe, is true for NP15. Much of what we see in this story has nothing to do with the actual rule. It is included because it is important for our understanding of the Vinaya and this was a convenient place to put it. In fact, it could well be that the events described were actually related to the laying down of NP15.
The actual part of the origin story that deals with the rule is only the very last part. The point, I think, is that because the monks were discarding their blankets, the Buddha laid down a rule to ensure the blankets were not simply wasted. In this case I would say the match between the origin story and the rule is arguably quite good.
All quite interesting, I think. I believe we may well be seeing traces of authenticity in these origin stories. All of which is a far cry from how many academics would view this. And if we can find signs of authenticity in such secondary material, this supports any claims of much greater authenticity for the primary sutta and vinaya material.