What is the source of their right to seek an empire in the first place? At root it's the social contract of the society whose members they seek to enlist.

The source of the implied responsibility of beneficence is the fact that without assuming such a responsibility the acts of a corporate empire are likely to be destructive to the long term interests of the society. By (for example) controlling the social network, they leave members of the society with limited choice not to participate, and thus the implied responsibility of beneficence in exchange for that removal of choice.