Yeah the clause is fine, anyone who disagrees with it is just scared of having their claims taken away.
But if you can't put in the time to play, you don't deserve those claims. Simple as that.
Don't lump everyone who disagrees into a single blob, especially since it isn't true.
I personally have little fear of having any canon claims of mine taken away or unadulterated, and I disagree with this clause. I don't typically prohibit people from using the canon shadow imitation techniques and in truth have very little canon anything to lose in this discussion. I just tend to tell "newer to the dark side" folks who want to use them to at least claim to be a Nara of some sort, and that's a request not a demand.
Read that first part of it again, "If a canon claim belongs to someone" not "If a canon claim nebulously belongs to a group of people simultaneously whilst none of them actually currently have ownership."
Kayenta and the Mist argue that a group of people, not just a single individual, has ownership of the swords. Bocchiere and the writer(s) of the clause would/are disagree on that. "Ownership" is very different in that discussion:
A) Ownership may be plural, and does not necessarily entail championing of the item in question.
B) Ownership is singular, and does entail championing of the item.
B is very convenient and is the convention used for the tailed beasts; there can only be one champion at any time. A is also very convenient, and is generally used when there are multiple users of jutsu (lightning release armor) where there is not much centralization/order for ownership, but many claimants.
Other examples of A are:
Hiden (secret techniques of the clan sort)
Restricted Techniques (Edo Tensei, Hiraishin, etc.)
Special canon tools (three pronged kunai that aren't as popular as they used to be)
The examples for B are numerous, and I don't really feel need to be emphasized.
The problem with B is that there has to be an Owner/Champion at all times, otherwise the item in question is in limbo and claimable by anyone. The problem with A is that if one of the claimants is snatched/brainwashed/ otherwise, then the claimed item/jutu can switch to a new claimant without the permission of the remaining claimants.
The Mist Swords fall under A, to be technical. However, to more control the distribution of the swords, the Mist implemented a scroll/seal/method that would functionally keep the swords from leaving Kiri's possession unless one were to go through the rigorous process of becoming a Ninja Swordsman and then defecting, finding a way to keep the sword from returning to Kiri's possession upon declaration of the defection.
That's kind of RP intensive, especially if other logical ways of getting the items are not available for, reasons.
Anyways, I have somewhat deviated from my point what with my discussion of the Swords. The definition of "owner" in this clause needs to be expanded to include multiple owner situations and not just selectively one-owner situations, because not all claims have only a single owner. Additionally, the owner(s) need to actually be inactive for this particular clause to be evoked, not merely the claimed thing not being used.
Another, separate clause can be included to deal with acquiring the claimed (most of the time it is either get it from an owner or claim it when there are no owners/claimants), but for this one I think it needs to be considered that not all claims have a single owner.