So? Talking at all helps Mafia identify who to kill; does this mean we prevent talking at all?
The thing is, the Mafia has one win condition: Eliminate the Town. It really doesn't matter who they kill off, but it does sort of help to kill off the right people. These desirable kills are not necessary in the large majority cases, and generally, to the Mafia, three kills are three kills. It doesn't matter who is on the receiving end of said kills. However, to the Town, they are trying to kill off the Mafia. These are a specific group of people representing a small fraction of the overall playerlist. These people are not interested in helping out the Town; this sometimes shows in their posts. The Town's end goal is to eliminate the Mafia. If one of the best ways to find out who they are is to get them talking, why keep the talk quiet at Night? At the end of the Night, the same amount of people are going to end up dead if there was no talking than if there was talking.