They used to have a noise rule that they enforced. A QB could step back from the center and the play clock would stop. He could wait for the crowd to simmer down before he ran the play. But it wasn't real effective. Offensive players don't like standing around motionless. It interrupts their train of thought.
The NFL is taking a quite different approach nowadays. They are more concerned with enhancing the game day experience for fans and allows teams to encourage their fans to make noise and to be a part of the game. If they really want to stop it, the first step would be to not allow stadium announcers holler into the PA system "Thirddddd Downnnnnnn!", scoreboards encouraging fans to make noise, and so on, which happens in every single NFL venue.
This is not something that is limited to Seattle. It happens in every NFL stadium in the country. We just do it better than anyone else.