Backstory radio (public radio program where three professors who used to teach at UVA examine a topic through three centuries of american history) did a recentish program on the police and interviewed Alasdair Roberts.

the interview segment introduction: "Legal scholar Alasdair Roberts talks with Peter Onuf about the beginnings of municipal police departments in the U.S., which were organized not to stop or solve crime, but to maintain public order."

Essentially the broad strokes on the claim appear to be a credible view that marxist historians have teased out. http://backstoryradio.org/shows/serve-protect-2/

there is a lot of simplification in the article and a clear ignoring of the role the police force integrations were correlated with rising status of ethnic groups (and the whole century timejump) but the basic historical narrative shouldn't be labelled as crankish and ignored (which is what i took your question to be about at its core)