Why is the spirit bear/Kermode bear a subspecies of the American black bear Ursus americanus?
It is because somebody wrote a paper that described it as such, and the paper was published. I don't mean to be flip. Its just that taxonomy is done through publication, not committees (although the peer reviewers could be considered a kind of committee). Subspecies are less stringently regulated than species, and this subspecies does have a particular unique set of gene frequencies that occasionally (as you mentioned) show up in the phenotype. The fact that it has been culturally recognized as special may also have something to do with its subspecies status. A subspecies need not be reproductively isolated from its parent species, and need not have any fixed characters.