Interestingly, the U.S. Board on Geographic Names has a longstanding policy dating back to 1890 to discourage the use of possessive forms and especially apostrophes in place names. This is discussed in their FAQ (question 18):

I have heard that the use of the apostrophe “s”, such as Pike’s Peak (Pikes Peak in the database) to show possession is not allowed in geographic names, so why are there many such entries in the GNIS Database?

Since its inception in 1890, the U.S. Board on Geographic Names has discouraged the use of the possessive form—the genitive apostrophe and the “s”. The possessive form using an “s” is allowed, but the apostrophe is almost always removed. The Board's archives contain no indication of the reason for this policy.

The USBGN’s editorial guidelines expand on the matter:

Apostrophes suggesting possession or association are not to be used within the body of a proper geographic name (Henrys Fork: not Henry's Fork). The word or words that form a geographic name change their connotative function and together become a single denotative unit. They change from words having specific dictionary meaning to fixed labels used to refer to geographic entities. The need to imply possession or association no longer exists. Thus, we write " Jamestown" instead of " James' town" or even "Richardsons Creek" instead of " Richard's son's creek." The whole name can be made possessive or associative with an apostrophe at the end as in " Rogers Point's rocky shore." Apostrophes may be used within the body of a geographic name to denote a missing letter ( Lake O' the Woods) or when they normally exist in a surname used as part of a geographic name (O'Malley Hollow).

So, the official name of U.S. places, even when it has a genitive s, never has an apostrophe, by fiat of the U.S. Board on Geographic Names, which is a part of the U.S. Geological Survey (USGS).

Edit:

In Australia, a similar board—Committee for Geographical Names in Australasia—has made a similar ruling about apostrophes in place names:

4.12. GENITIVE APOSTROPHE

In all cases of place names containing an element that has historically been written with a final -’s or -s’, the apostrophe is to be deleted, e.g. Howes Valley, Rushcutters Bay, Ladys Pass. This is to facilitate the consistent matching and retrieval of placenames in database systems such as those used by the emergency services.

I don’t know about any official rule in Britain, but the tendency is widespread there. The Wikipedia article about apostrophes mentions these:

There is a tendency to drop apostrophes in many commonly used names such as St Annes, St Johns Lane, and so on.

In 2009 a resident in Royal Tunbridge Wells was accused of vandalism after he painted apostrophes on road signs that had incorrectly spelt St John's Close as St Johns Close.

The second sentence there includes a link to this article about a man in England who decided that the tradition of excluding apostrophes from place names should be reversed by painting in apostrophes onto public signs, and was (correctly, IMHO) branded a vandal. If I were the reporter for that story, I would not have been so forgiving and deferential to what the perpetrator considered to be “correct English”.