Wealth is power. Inequality of wealth is inequality of power. The more wealth one has, the more people one can hire, and the more power one can exercise over those workers, even when one is getting ready to fire those workers:
Capital One employees didn’t talk to us as they exited the building today. They were advised not to speak to the media or they could lose their severance pay. But we’re told that employees must attend a meeting September first to get their 90 days’ notice [Angela Kennecke, “Capital One Employees Get News of Layoffs,” KELOLand.com, 2015.07.23].
Capital One’s wealth gives it the power (effectively, though not justly) to restrict its less wealthy employees’ liberties. That effective power is why inequality of wealth is bad for democracy.
We form a social contract to ensure that everyone can eat. We form a democratic social contract to ensure that everyone can speak. That one rich person (here, a corporate person!) can force hundreds of other persons (real, natural, flesh and blood voting persons!) to refrain from public discussions of a matter of public import is unjust and inimical to democracy.