Particularly in America, the police exist primarily to enforce class/racial divides (this is not hyperbole -- one of the best quick introductions I know of came out quite recently and is a very worthwhile read or listen: https://www.npr.org/2020/06/05/870227945/nprs-history-podcas... ). This is fundamentally contrary to the politics of a labor union, which is to empower those who perform labor to negotiate on equal footing with those who would claim the bulk of the productive output of that labor.
Both would fall under the aegis of "collective bargaining", but the social aims of the two couldn't be more different: the maintenance of a stratified class system vs the dismantling of same.
Both would fall under the aegis of "collective bargaining", but the social aims of the two couldn't be more different: the maintenance of a stratified class system vs the dismantling of same.