I have known some people that claims to be Japanese (and talk it) but they aren't. They do it to get better chances, not be bothered, etc.
It is like:
"<I am sorry.> <I don't speak any English.> <Goodbye.>"
But then you see him/her around talking English, WTF? that is denying your language and nation, I don't think that is right. So if you see that point, it is bad.
But about putting only the J. I think that some Americans / Non-Japanese do this because they simply prefer Japanese parties for their experience in the game, or they are simply ????ers.
Personally, I use the JE because I want both parties. No matter English or Japanse.
So in short words, what I mean is that like many already said here, it depends on the person, the game version, etc.
Not everyone around with J does it for the same reason, nor those with JE.
Of course, this is what I think, so don't blame me if you don't think the same way
