'In purity'? What on earth does that mean? Sounds like prescriptivist nonsense to me.
That means that it has been spelt with a z for centuries. Then some English smartarse decided that it should be spelt with an s as it is in French, which makes no sense to begin with because it is pronounced with a z-sound not an s-sound. Then the English intelligentsia started spelling it with an s to try and make themselves look clever, from where it spread to the aspirational classes. The Americans retained the proper spelling though, so now most people in Britain believe that spellings such as realize are Americanisms, which they are not, and avoid them for some chauvinistic reason I suppose.