It wasn't United staff who assaulted a customer, it was airport security staff. The article doesn't say the US won't punish O'Hare Airport over the incident.
United is not a small concern, they have lawyers and PR folks that a small restaurant doesn't have access to.
> We did not review the actions of the security officers of the Chicago Department of Aviation because it is not DOT's role to investigate police conduct.
- http://www.flyersrights.org/wp-content/uploads/2017/09/May-1...