First of all, there are 2 English versions of the client:
1) enGB - the British English version of the client, aimed towards the European market
2) enUS - the American English version of the client, aimed towards the US market
Oxford English is studied in most schools across Europe and the US spelling (center-centre; theater-theatre; color-colour, airplane-aeroplane; meter-metre; etc) is frowned upon and even considered a spelling mistake in exams and tests.
The question is - why does the enGB version of the client use enUS language files? The children that are playing the game, especially the non-native speakers, can learn the 'wrong' spelling of the words. The native speakers in England generally frown upon non-british spelling in newspapers, magazines, books and even in forums and message boards. Why is WoW an exception?
Most common typos in WoW:
1) Honor. Honor, honor, honor everywhere. Valley of Honor. Honor Points. Honorable kills. Honor Hold. The correct UK English spelling is honour.
2) Armor. Same as honor. Correct spelling: armour.
3) Defense. Should be spelt as defence.
4) Traveled, canceled. The correct spelling of the past tense of these words should be with double L: travelled, cancelled.
I'm not here to bash the American English, I respect it and I personally spoke it in informal situations. However, in formal situations I'm required to use British English, otherwise I will sound silly to them.