"Johnny is a Jew," said in a casual, informative tone and "Johnny is a Jew" said in a suspicious, derisive tone with emphasis on the last vowel.
If 'Jew' in the last instance is offensive because it is prohibited, then 'Jew' said in the first instance also should be offensive because they are the same word. On the other hand, the same word can have very different meanings (that is, have different semantic contents) depending on context which would explain why it is not offensive but informative in the first instance but used as a slur and thus offensive in the second.
Also consider that many slurs also have non offensive meanings and are commonly used to mean other things than the slur. Consider 'chink', 'cracker', [in British English meaning cigarettes] 'fag' and so forth. Now 'fag' is becoming obsolete partly because of its association with the slur for homosexuals much like "niggardly' is mostly obsolete because its superficial similarity in sound and spelling with 'nigger'. But there still are plenty of examples of the same word being used non offensively to mean something common and its being very offensive at the same time because it is also used in other contexts to mean a slur for some group and this would seem to counter the prohibitionist view. I also think there is a "cart before the horse" issue that can be raised against the prohibitionist. They cannot explain why some words are prohibited rather than others.