From what I've heard, RMT itself is big in Japan as well as here, it's just advertised through third-party sites rather than in-game. At least ad space on fan sites is less disruptive than in game /tell spam. But I think that difference is why SE doesn't take the spam seriously. Of course they have GM teams on this side of the world that see the spam we get, but the devs actually making decisions about how to handle it don't see any /tell spam issues because they're playing on JP servers. They only see the issues RMT causes to the in-game economy, so that's the only part they care about fixing. (And that's the part that's hard to impossible to fix. The spam would be the easy part to get rid of if they'd bother to do so.)