Imagine a $2T company. One day CEO decides that his company should stand for only Republican values, donate them handsomely, buy newspapers for them, start TV channels for them, only offer employment to republican leaning persons, introduce new laws through lobby... All of these is in fact legal. When a company becomes too big, a single person can control its resources towards hurting political system, general public good, tilt opinions and so on. You are then relying on this one person to knowingly or unknowingly not make a bad move. Most often, they will.