Knowing Nintendo, it's probably a measure to keep bullying and explicit messages from being sent among its' users. A lot of their games are aimed at a younger audience so they probably want to avoid any negative PR that could harm their customer base. For instance, parents expecting their video game consoles to teach right from wrong and getting mad at the company when little Timmy starts dropping F bombs everywhere.
Plus isn't Japanese culture a little more conservative? They probably value their users experience more and don't want people negatively affected by other people having the freedom to say whatever without regard to consequences.
While Nintendo is certainly behind when it comes to online functionality, they know there are a billion ways to communicate today so it's not going to be a system killer if they don't include it.