It's been stated repeatedly when the subject of chat silences has come up.
When a report is made by either a player or a GS the chat logs are reviewed and a silence is implemented if warranted.
I can't say for sure if those steps are being followed but that is the process that we've been told is followed. Perhaps some of the silences are unfair. I'd wager most are not. I don't have much in the way of experience with them.
I can say that I know more than a couple of people who have been silenced that yell and scream about how unfair it is in public but in private acknowledge that they were being awful and revel in how many silences they've racked up.