Why? For a country that does not speak English so well what is this big concern about the pronunciation of English? What is Accentism? It is merely people having racism on someone's accent hence the word accent ISM.
Although Japan is not good at English as a whole country. If they do learn a language they want to learn it in "proper" English. Don't misunderstand, I am a true believer in getting the point across as being the best form of communication in any language but Japan has a special heart for Westernized English and British English. I think it is more because of movies, CDs and DVDs. Other accents are hard for them to catch clearly so they have a hard time understanding. Japan does not use images to understand, they use words. That is the same with Kanji. Kanji is a character and not an image that they use to form the words for other languages. For Example, Dog together is a word or a character to them not the sound of each letter to form the image of a dog. For this reason, if the sound is off, it is confusing for them. Does this make sense? Just like their language, it is learned a specific way and no other way. Those from other countries will have to adjust their sounds in school. Whether you are from Australia, Indian, or an African nation. You must learn how to adjust your pronunciation in school. One way to help you is to learn basic phonics. I have a Youtube video that can help you.