One might rationally think that whenever a police officer is reasonably aware of a criminal activity occurring, that they have a Constitutional responsibility to arrest the perpetrator, or at a minimum to make contact with the alleged perpetrator, for the safety of that community, and in good prudence of performing their policing duties. After all, the police are specifically engaged by the public purse to perform their duties on behalf of that society, of which their constituents are the public, at large; in which the very purpose of having a police force within communities in the first place, is so that they effectively thereby replace all previous forms of policing type organizations, such as vigilantes, militias, individuals, or privately financed security, so as to maintain law and order within a given community.
In other words, the people that make up communities are instructed to not take the "law" into their own hands, but to contact the police for the enforcement of those laws, of which, the structure of the deal for the people is that they have essentially contracted with those police officers through the auspices of the governance of that community, to effectively police their community. Therefore, to the degree that police officers perform their duties, in an evenhanded manner and of being no respecter of persons, is to the degree that such policing is fair and equal.
However, when police officers, for whatever reason, are in essence, co-opted by some important members of that society, or by the prosecuting arm of the state, or through the authoritarian structure of the police organization, itself; or through any other means in which those police officers are now performing their duties in a manner in which some are served, whereas, most are not, signifies a breach of duty to the people, and clearly becomes a situation in which rather than the police being utilized in a manner to uphold the law for all, they are, rather, being selective in the enforcement of that law, so as to be, in essence, of benefit to some, and of possible detriment to many.
Thereupon this creates the rather dubious problem that infects so many of our communities, in which, the law as exercised by the policing arm of the state, is unevenly applied, so that some are persecuted to the ends of the earth, whereas others that are also committing criminal acts are simply left alone. One might reasonably think therefore that any time that a police officer arrests one person, but thereupon allows another person under somewhat similar circumstances, to remain free, that such must be a Constitutional violation, as clearly justice is not being equally applied. Unfortunately, though, despite what many people believe, the fact of the matter is, in absence of a specific arrest warrant, the police are under no obligation to arrest anyone, but are permitted, in effect, to arrest whoever they so arrest, per their discretion.
So then, in a land in which the laws of that state, are effectively unevenly and arbitrarily applied, one can expect the sort of thing that we see all of the time in America, of which, some specific citizens suffer the enduring indignity of arrest, seizure and incarceration; whereas many others who are engaged in their own criminal activity, but are well placed, and well connected, are simply let be.