It does seem odd, considering that in contemporary cultural anthropology
"Bongo-Bongo" invariably represents a purely fictitious (presumably
African) tribe.  Often it's given as a sort of ironic metacommentary,
critiquing anthropologists who come up with obscure counterexamples to
posited cultural universals.  The earliest usage I find on the JSTOR
database of scholarly journals is from 1962:

        Review of _Femmes d'Afrique Noire_ by Denise Paulme
        Robert F. Murphy
        American Anthropologist 64:5 (Oct. 1962), pp. 1075-1077.
        [Women's] status contains within in its own compensatory
        features, and if the male anthropologist cannot see this
        in his hearth, how can he detect it among the Bongo-Bongo?

A better known example is from Mary Douglas (1970):

        _Natural Symbols: Explorations in Cosmology_
        By Mary Douglas.  Barrie & Rockliff, 1970.
        It serves to counter the effects of Bongo-Bongoism, the
        trap of all anthropological discussion.  Hitherto when a
        generalization is tentatively advanced, it is rejected out
        of court by any fieldworkers who can say: 'This is all
        very well, but it doesn't apply to the Bongo-Bongo.'
        (text available on Amazon: <http://tinyurl.com/2qt6l>)

Last time "Bongo-Bongo (Land)" came up here, it was in the context of
Tory minister Alan Clark's notorious usage [1].  I suggested in that
thread a possible connection to the song "Civilization (Bongo, Bongo,
Bongo)", recorded by the Andrews Sisters with Danny Kaye in 1947
("Bongo, bongo, bongo, I don't wanna leave the Congo...").  This in turn
was perhaps inspired by a song called "Bongo on the Congo", lyrics by
P.G. Wodehouse and music by Jerome Kern, which appeared in the 1924
musical "Sitting Pretty" [2].

I think Wodehouse might have been responsible for inventing the mythical
Bongo-Bongo, but not in the musical number (though I haven't found the
lyrics for it yet).  In two short stories that first appeared in 1932
("The Story of Webster" and "Cats Will be Cats" aka "The Bishop's
Folly"), Wodehouse introduces a character named Theodore who accepts
"the vacant Bishopric of Bongo-Bongo, in West Africa."  I could imagine
British social anthropologists reading Wodehouse and then applying
"Bongo-Bongo" (ironically, of course) to their own discipline.

[1] http://groups.google.com/groups?th=17e7c8fc4d782ea2
[2] http://wodehouse.ru/musical.htm
[3] http://wodehouse.ru/49.htm
    http://tinyurl.com/2frbt ("The Story of Webster" on Amazon)