Away (post & fold) question in tournaments

Why do we allow people to join a tournament that never play one single hand through out the whole tourney. They stay in the away(post & fold) mode the whole time. Can they be booted from the tournament lets say after six deals? Seems silly to me that they sign up and not play. I played in a tourney yesterday with nine people at the table. Five of us played, the others were in away(post & fold) until they ran out of money.