Reading between the lines I would say that the logo of the WHO has been designed by a professional graphic artist who may well be more familiar with American English. The American spelling (with a "z") has then been juxtaposed to the colorful logo as a matter of course. Evidently this has been overlooked or ignored by all those responsible for writing the text which is, for an international organisation like the WHO, quite an undertaking involving WHO policy makers, lawyers, wordsmiths and sub-editors. The British English of the text has to be read with the logo's American English spelling of organization with a "zee". This is not going to shake the foundations of the WHO or, for that matter, speakers of AE or BE. I think it is quite acceptable to use the acronym WHO for the main body of text and communications.