The reason that police officers should avoid constitutional violations is that the express purpose of their institution is to protect the constitutional rights of citizens. In modern times, it's easy to think of the police as an entity that is meant to "punish," but the central idea to a police officer's job should always be as it is expressed in their official motto: "to protect and serve." One of the guiding principles of the United States is that branches of government and individual agencies should never have unrestricted power to do whatever they please, and both state and federal law enforcement agencies are no exception.
Even criminals, prisoners, and those accused of crimes have rights. This is one of the guiding distinctions to be made in an ideal democratic society. Lately, there have been repeated incidents of reported police brutality in the United States, wherein police officers have allegedly resorted to violence far sooner than they should under constitutional protocol. Besides the obvious problem it represents for victims of this violence, breach of protocol decimates the trust between police entities and their communities. Citizens will be far lest likely to trust or even call upon police aid without assurances that they will be acting within their limitations as dictated by the law.