I am not sure why abuse is so consistently tolerated in the U.S. legal system. I suspect it is because, like the supremacy of property rights, it is a remnant of legal slavery. To a certain extent, every remnant that persists provides evidence that the abuse of humans by their own kind was not all bad.
If it is right to beat children into submission, if they do not learn well or obey as required, then such behavior is sanctioned currently and in retrospect. If owning property is a privilege, then the exclusion of some people from the necessities of life cannot be wrong. Abusive police are not descendants of slavery; they are the remnants of a stratified social system that rests on violence. Not only is the physical abuse of a civilian classified as a misdemeanor; simply questioning the authority of a public servant is classified as an assault on an agent of the state.