So, if I went around in blackface and genuinely thought I was acting like a black person and was having a laugh at how black I was being, that isn't racist?
I don't know about blackfaces, but there are numerous cases of imitating Black English (I'm talking precisely phonetic variations, although Black English as a dialect is NOT limited to them). It's especially wide-spread in the hip hop fandom, with people genuinely using the AAVE variants of English words among everything else. If that's how they mock black people, how come they're hop hop fans at the same time? Does not compute, sorry.
Like ZoaKaizer said, such accents are used not [only] because they sound funny, but also - and that's a solid reason - because they have made their way into popular culture, are associated with a certain pop culture phenomenon (hip hop for Black English and anime for Japanese English) and end up being deemed stylish. It's the case when istead of contempting the said phenomenon and undermining its reputation, such imitation - ranging from serious intentions to a concept called "affectionate parody" which seems to be foreign to you - ends up actually promoting it. That's why people keep imitating those accents, and that's why neither African Americans nor Japanese raise cain about it. They choose to seek benefit in it.
That, and a couple more points. First, I'm honestly wondering how in your view it can murder Japanese language when the language itself has a huge and firm base of native speakers who operate it in everyday use. Not to mention that the "accent" as you call it has long been contributing to Japanese vocabulary. I saw a Japanese textbook (written by a Japanese author, I should note) that was called "You already know Japanese!" - and listed this very "accent"... only he called it "a list of anglicisms in Japanese that a foreigner can use in communication".
Meaning the Japanese people are quite at ease with the way they pronounce English words, seeing as they include them in their dictionaries. So if it's no fault to them, how exactly imitating it is classified as mocking them?
Second: You know, there are both living and late legendary actors in Russia who have/had TONS of jokes told about their personalities, quirks and weaknesses. But they all remain loved by the very audience and colleagues who tell jokes about them.
TL;DR: There are different types of laugh; try a bit harder in identifying the kind ones.
lol, just when I thought we were done with lengthy debates, they come back from a completely unpredictable direction. I hope it'll end before we post several pages and get asked to move to PM? 
Of course, if there's only one extreme way to prevent such a lengthy discussion, I can do it for the sake of others 
Here goes... (*deep inhale*) SORRYPONTIANAK-SAMAWEWERECOMPLETELYWRONGANDYOUWERECOMPLETELYRIGHTANDYOU'VEOPENEDOUREYESATTHETRUEMEANINGOFRACISMANDMOCKERYSINCEYOUARETHEBESTEXPERTINSUCHMATTERSWECANEVERHOPETOFINDANDWEACKNOWLEDGETHEERROROFOURWAYSANDSWEARTOREDEEMOURSELVESBYSTARTINGTOLEARNSWEDISHIMMEDIATELY! 
whew... OK, did it help? 
