The issue is simply.
If we have a generic type name that is used globally such as Kudu for the species, and a local community only has the subspecies, then they should be allowed to use Kudu locally for the subspecies if that is the name they know it by. Calling it a Greater Kudu will result in lots of good data being misclassified as Kudu because local users will know the subspecies only by the name Kudu and select that name. And quite honestly the voting system in iNat is so primitive that the name will be stuck on the species, even if lots of knowledgeable people try and classify it to subspecies, when less informed people will go for the name that they know and thus diminish the value of the observation.
The only way around this is to have to laboriously explain on the observation to each new observer that iNat has this funny system whereby we are not allowed to call our Kudu with the name Kudu that we use, because someone else has decided that our name must be used "at a higher level" and that therefore you must in future please call it a "Greater Kudu" even though no one in the country ever calls it this, because that is how well iNat caters for our local needs and you can always go and use some other site if you dont like it. Besides stop being such an ignorant and lazy lout and - to quote the words of the iNat curator's guide: "(if you) want to take it to subspecies (you) can just learn to use the subspecific name (or better yet the scientific name)." So there you are! If you have a common name for a subspecies, it is time that you grew up and learned the scientific name, otherwise there is no place for you here on iNat. Also, on iNat ID to species is good enough: we dont really want you to make IDs to subspecies: who cares about them anyway: one of the INat programmers has stated "and I think encouraging a species rather than subspecies taxon is probably for the best, since a species-level ID is not an incorrect ID". So you mompara: it is a Snake: surely that is good enough for you: after all a genus level ID is not an incorrect ID, nor is a family ID, nor is an order ID incorrect - so what the hell are you doing on INat anyway - its Life and what do you want to know more for? Sheese - I suppose you will want to know next if it is Peter, or Tony or Charlie? Just go and ask ouma: she will tell you all you want to know.
Why was a decision made that duplication of a name at one level, was not allowed for a specific place at another level?
I think that this is just bad programming: it appears that the problem is that if we decide to call Tragelaphus strepsiceros strepsiceros as "Kudu" in region southern Africa, then iNat automatically includes that name at that level (subspecies) across the entire platform. Yes, it might not be the preferred name, but it still comes up in all the searches for the subspecies. So consequently Kudu will appear globally on any search of the subspecies. The fact that this is technically correct and that any Greater Kudu is also a Kudu, by definition seems to have escaped the logic.
A far more logical approach will be to restrict a local name, to that region, and not copy it across as a global name if it is a regional name. But the programming does not have a "global" region: any regional name also automatically gets elevated to the global list. That is the crux of the problem and also the way around this problem.
On the other hand if this is not the issue, it is purely a matter of someone deciding that any common name may only be once in the iNat dictionary - restricted to one taxon, then this is ridiculous.
* What is iNat going to do with the names we want to load that are duplicated by different communities in southern Africa for different species? So there are going to be lots of duplicates anyway. iNat is going to have to live with this problem as more and more countries come on board and start wanting their own local names. And just like not every country speaks American, not all counties have different languages, or even only one language.
* And wait until iNat hits the problem of the Tswana being upset because you only allowed the Swahili name, when in fact the exact same name is also used by the Sotho, and the Pedi, and the Shona; why have you only allocated it to one language and why did you chose that language group, and why can they not have their names recognized?
* And you still have to discover the joys of species named for some medicinal or use value. A name that will apply "generically" to half the species in the genus, but not the other half. So 15 species in the genus will have the same name, but 10 will have another name: so that elevating the name simply to generic level will be wrong. (I dont know any examples, but I imagine that sometimes unrelated genera may have members with the same name, because of some morphological or use similarity).
In short, trying to be proscriptive with Vernacular Names is not going to work! Sooner or later you are going to have to relent. it would make more sense to fix the programming and make the system more robust to the fickles and vagaries and sensitivities of Common Names. After all if you want only one name, that is what the scientific name is for.
Just for the record, I - as a scientific researcher who uses Citizen Science data - and the South African National Biodiversity Institute (that is legally mandated to manage biodiversity information for the country from all sources), would prefer identifications to the finest level that they can be done, even in cases of taxa that have varieties of subspecies, bearing in mind that sometimes "the finest possible" may be a higher taxonomic rank!