Just because he is moving boxes doesn't mean he isn't providing value as a CEO. Presumably he is learning things that will empower him to improve the company somehow, which seems like a valuable thing to have your CEO do. I'm sure Mr. Bezos is smart enough not to go AWOL for a week if there were more critical things to attend to.
Also don't underestimate the value of improved employee morale and good publicity. Your company is vastly more productive with happy employees, because they will work harder and go the extra mile for something they believe in.
This means that employees and junior managers get a chance to evaluate their bosses as much as the bosses evaluate them. I don't think Bezo's action shows a lack a faith in lower management but rather a "trust but verify" approach.
Of course it might be a more strategic approach to finding out how stuff really works on the front line. Given a big enough management structure the "on the floor" reality might not filter up to the board room correctly. Remember the game of gossip? That effect plagues large organizations.