e.g. a doujin cover has ARTIST and CIRCLE as separate tags (sorry, poor wording).
Circle tags were known before many artist names, so I would guess that they have been aliased for convenience (easy search) and so that it doesn't break tag subscriptions. I believe that in most cases, circle : artist is a 1:1 relationship, so that makes sense.
You use SNK as an example, but it was decided many years ago that company tags such as SNK and capcom should not be used for single-game images. Instead, you should use them for crossovers with company_connection.
Moreover, SNK is a copyright tag. Do you suggest ARTIST and CIRCLE to be both artist tags? (of course, having an unrelated copyright tag would be bad) The only circle I know of that has 2 artist tags on each post, 1 for the artist and 1 for the circle, is dakemakura-koubou. I don't know why. If we added qualifiers to the other 4 artists in the circle, I could search for *(dakemakura) to find all of them. EDIT: I guess this just exists because "find artist" doesn't handle multiple artists uploading from the same source very well.
Separating artists and companies/doujin circles is not necessarily a bad idea, but if we are going to do it then we should do it properly. Just making them into a copyright or artist tags is not right and will bring more confusion than convenience. By "properly" I mean creating new tag category/type that would also have its distinct color, like it's done at yande.re.
Whether this is worth the effort is another question, though. One advantage I can think of is that we already have several company tags that are currently marked as copyrights, so this would allow to properly distinguish them.