First off, to start my explanation on this let's remember that the game is 9+. In order to earn money from a younger, more gullible audience, the server operators must ensure that things are set in order to prevent anything that would make the age limit have to be raised. That is why certain things like swear filters and GP uploads are added. Now if you look on certain games which lack these, you may see a little thing near the rating such as "Online interactions not rated by the ESRB" or something like that. This means that due to the lack of swear filters, or addition of voice chat, the ways you play online can't be given a rating. An admin can not simply make a child "un-hear" a swear word, and that is why there is no voice chat.