Ars points out the increased scrutiny of “the role that MediaSentry plays in the RIAA’s legal campaign and whether the company should be licensed as a private investigator.” In one case, the RIAA argues that MediaSentury is actually a “copyright investigator,” and thus not subject to laws governing private investigators.
Private enforcement of copyright law has a history in the English Stationers’ Company.
During the Tudor and Stuart periods, the Stationers were legally empowered to seize “offending books” that violated the standards of content set by the Church and State; its officers could bring “offenders” before ecclesiastical authorities, including the Bishop of London and Archbishop of Canterbury.
Wikipedia isn’t the best source for the story of the Stationers. I highly recommend Patterson’s Copyright in Historical Perspective for a convincing argument of the Stationers’ role in copyright history. Private enforcement of the law isn’t something we do much of in the U.S. (that I’m aware of), so we need to use history as a guide wherever appropriate.