Yves Smith has a devastating article on the furor regarding United Airlines ejection of Dr. Dao. She argues that neither the press nor United did a good job. The law states that a paying passenger with a confirmed seat who has been seated cannot be removed. Very few, if any, articles in the press point this out. The press seems to be convinced that United had the right to remove Dr. Dao. Further, the press reported that the doctor reboarded the plane; there is little proof of that. And, there was little mention of the possibility of a concussion, which did occur.
United also was contravening the law when they gave preference to their employees over people who had reserved confirmed seats, in violation of 14 CFR 250.2a.
If you are interested in this situation, you really should read Smith's article.