Laws, industry regulations, corporate policies and police enforcement are jumbled together, collaborating in a corporatist manner.
(A) The airline has policies and business practices such as overbooking & "must fly." These save money, get staff to their required destinations, whatever.
(B) Police enforcement is a required for these business practices to actually be practiced.
The only reason they can practically have this involuntary removal by lottery policy is police are willing to enforce it. If police of any city/airport refused to execute these corporate polices, the policies would have to change.
That's ultimately who outrage should be directed at, the police. Why did they come in the first place? Had a crime been committed? Passenger safety endangered? Police were summoned by a "private" company. They were informed that corporate policy required them to drag a passenger of the plane. They said OK, I guess that's our job.
I'm not as concerned about United or anyone else having nasty policies. Many companies do. I care more about police enforcing those policies. There is no consumer protection against police action.