Kamiya reads all of his Twitter fanmail, and responds to almost all of it. Trolls realized that this is an easy way to troll him (and easy trolling is the pinnacle of trolling), so they regularly send him crap. And he regularly tells them to fuck off, or types some other effortless dismissal of what they just said. That's just how he handles his Twitter. He could choose not to feed the trolls, but he doesn't.
Kotaku wrote a terrible article about him, and Kamiya responded with a simple "Ah, douchebags."
A Kotaku writer asked him if he had even read the article (because nobody who actually read the article could think the writer was a douchebag, not even the subject of the article), and he asked if they eat shit. That pretty much ended the issue nicely.
Then he said good night and (deliberately) muttered something in Japanese about baka gaijin and posted a smiley.
One of his fans said he didn't have the balls to say "fucking stupid idiot foreigners", so he (tongue in cheek) called his fan a fucking stupid idiot foreigner (singular).
I really can't see anything inappropriate in his behavior, besides some foul language, which is already a regular part of his Twitter.