Well, I think it's just good fun.  People here 
are anime fans, and a lot of people are fans of 
Japan in general.  I have met a lot of anime 
fans, and a great deal of them are actually 
studying Japanese and Japan in college. Many 
speak it well, my bf can almost speak it 
fluently.  No, they're not Japanese, but they 
love the culture and want to immerse themselves 
in it :).  I for one do not have a Japanese 
handle, nor do I make any pretense of speaking 
the language.  If you look around on YJ...you 
will see a lot of handles...and not all of them 
are Japanese sounding, hahah!  I also hear the 
English language is a big thing over there.  One 
of the guys I know went there for some time, and 
he said he saw a woman with a baby stroller 
walking down the stree wearing a t-shirt that 
said "circuit beaver" on it...  Eh?  Have you 
ever been to the Engrish site?  Now there's some 
hilarious English for ya.  I've heard they like 
how our letters look, so they print random words 
on things.  I dont have a problem with it at 
all.  I figure why not, we share the world, we 
can share languages too.  If I went to a Japanese 
forum and saw names like Mr Magoo and Daffy-kun, 
I wouldnt think a thing of it.  They're fans of 
our cartoons, cool!  That's what Id think.  Just 
my POV!  

BTW...I've actually thought about this myself 
many times in the past, I just dont say anything 
for fear of offending.  To each his own.