Yes, it is true that police abuse inmates. There are even cases of people, such as Sandra Bland, who die after being taken into police custody. Although some of these cases are ruled suicides, they occur under very suspicious circumstances. For example, Sandra Bland had no prior history of suicidal ideation, was recorded having a heated discussion with police which led to her arrest, and was in fact a political activist against police brutality. All of these factors led to closer scrutiny of her death, and accusations that she had been abused and murdered by officers while an inmate in the county jail. The Sandra Bland case was unusual in the bright spotlight it placed on police brutality against arrestees. Police violence of this nature often goes overlooked, because people taken into police custody are seen as deserving of any rough treatment they receive.
Police are not concerned about the rights or feelings of inmates. They often work out there own violent tendancies on people that many of us considered to be the lowest of the low. This is especially true for police who come from a military background and have no people skills.
The issue of "police violence" has become a hot topic of late. There are instances when police are forced to use a certain level of violence to control a suspect and the situation, for their own safety and the safety of the community. However, police officers do not typically have much contact with inmates. Inmates are already convicted and in the prison system, so they would be interacting with prison guards.
Inmates are, by definition, in a correctional facility. Because they are not committing crimes on the streets and do not have interaction with the public, they do not interact with the police. Because the police never see these inmates, the police cannot be violent towards them. Correctional officers, however, may be violent with inmates.