Why dont the people of District 12 clap when katniss volunteers?

The fact the people of District 12 don't clap for Katniss symbolizes a final gesture of rebellion and respect. At least in this small way, the people of the Seam can rebel against the Capitol, can refuse to give in to the Capitol's control. It's a act of rebellion against the Capitol yes, but also a example of respect to Katniss. They won't let the Capitol mock her with its cruel interpretation of this [The Hunger Games] as simply a game, something to celebrate, to enjoy.