Apparently in curling, as in tennis, golf and some other sports, it is considered proper etiquette for fans to be silent while the players are in action. Various breaches of etiquette during the Vancouver Olympics have created somewhat of a kerfluffle.
Why exactly are the peanut galleries in these sports supposed to be quiet? It seems to me that noise per se shouldn't really be much of a problem for players in any of these sports. Sudden differentials in noise volume, however, would be a significant problem, and demanding that fans be silent exacerbates the problem -- it's a lot easier for a few obnoxious/ignorant fans to create a distraction when the rest of the crowd is silent than it is for a few thousand fans to suddenly start/stop cheering in unison.