This confirms what lexicographers have always known: that the laws of usage are relative, bending with the taste of the lawmaker. ... In the end it comes down to what is "correct" usage. We have no king to establish the King's English; we only have the President's English, which we don't want. Webster long a defender of the faith, muddied the waters in 1961 with its permissive Third Edition, which argued that almost anything goes as long as somebody uses it. (p. 40)
[There] are some confusing situations when it comes to apostrophes. For example, [A bunch of people] asked how to make a singular word that ends in s possessive. I know that this is a raging debate even at the highest levels of government because [two listeners] sent me a funny article describing U.S. Supreme Court squabbles over making the word Kansas possessive. Words such as Kansas that end with an s can be stumpers when it comes to apostrophes.
Is it Kansas's statute with an apostrophe s or Kansas' statute with just an apostrophe at the end? Justice Clarence Thomas wrote the majority opinion and prefers to leave off the extra s, referring to Kansas' statute with just an apostrophe at the end, whereas Justice David Souter wrote the dissenting opinion and prefers the double s of Kansas's statute with an apostrophe before the final s.
So who's right? ... Unfortunately, I have to admit that this isn't a hard-and-fast rule; it's a style issue.