1. It makes working in law enforcement a social identity equivalent to race.
There is already a quite clear since in which the social identity of being in law enforcement works as a collective badge of identity between officers, which often cultivates fraternal codes of unduly protecting others, hiding evidence of mistreatment, and blindly supporting the position of other officers simply because of the collective identity. The aptly named "blue code," or "blue wall of silence" cultivates a culture of corruption, wherein having a particular job alone is considered reason to silence honest reporting and cover up objectively immoral behavior, making the social identity constituted, in part, by a play-ignorant mentality in the face of investigation.
To act as if such a vocation bears, or should bear, the kind of deep identity significance and source of solidarity that someone's black racial identity plays is to treat an occupation with too much significance, in this case, the kind of significance and collective identity already present in toxic forms causing these repeated cover ups. Adding fuel to such an ethos--propagating a hashtag that is meant to engender the same deep identity interconnection as #blacklivesmatter--when that collective identity is already in a situation of terrific privilege and insulation from the forms of violence experienced by the black lives matter community, is not only disrespectful and unwarranted, but also potentially a reinforcement of this toxic culture of play-ignorance and mistreatment.
2. It misconstrues history. Blue lives have never lived under erasure, where their lives were considered less than, disposable, and constitutively hostile to order.
One of the most patently clear functions of black lives matter is to generate an awareness of the way in which black lives have continually been treated as having little or no value. In and through the colonization process of the West, black lives have functioned as property with mere use-value. Black sociality has always been considered dangerous to the social order--a social order built upon their denigration and disempowerment. A black life is a life under the threat of social death, a social life constituted by precarity and the potential of imminent death as most recently seen in the brutal murders of Philando Castile and Alton Sterling.
Blue lives have no analogous history, no precarious location from which their collective life needs recovery. If anything, the near divinization of policing and the recent increased militarization of law enforcement has functioned as a continued source of their over-empowerment, exemplified in the countless examples of enforcing the 'law' while operating in social spaces as if they transcend it. Blue lives are not lives under erasure, living under conditions similar to black life. It is the history of black lives not mattering that gives meaning to the hashtag. Blue lives have no such analogous history.
3. It misrepresents the present experience of the social life of police. For the most part, police move about social spaces with respect and honor. They are offered free meals and services, not suspected as thieves and free loaders. They receive funerals with parades and news coverage.
Blue lives have always mattered, present and past. Their experience of social space is (again, for the most part) one of profound privilege and deferential treatment. This is, of course, much more widely experienced in more affluent (read, largely white) communities, but when police experience of the opposite--say, in poorer and more socio-economically disadvantaged neighborhoods-- it is doubtlessly colored by deeply rooted painful experiences of mistreatment and oppression at the hands of law enforcement. This is not to justify such mistreatment, or say that police officers should not be honored and respected, but simply to say that they are. Their experience of the world is, in many ways, constitutively different than the lives of those who are judged as social dangers by their very appearance and existence.
It is profoundly misrepresentative and disrespectful to develop an analogous hashtag as if blue lives have an analogous experience of social life in America as black lives have. This hashtag is wrong in so much as it connotes that the lives of law enforcement officers have failed to matter sufficiently in the broader public consciousness. It connotes that there is some culturally collective making up for that failure that needs to be done by parasitically drawing upon the form and cultural import of the black lives matter hashtag. In so much as it locates police life in a situation analogous to black life, it is wrongheaded, harmful, and offensive.