A recent ruling from the European Court of Human Rights against Finland highlighted the crucial role ISPs can play in investigations and protection of the person. K.U. v. FINLAND (2 December 2008),
"The European Court of Human Rights has today notified in writing its Chamber judgment1 in the case of K.U. v. Finland (application no. 2872/02).
The Court held unanimously that there had been a violation of Article 8 (right to respect for private and family life) of the European Convention on Human Rights concerning the Finnish authorities’ failure to protect a child’s right to respect for private life following an advertisement of a sexual nature being posted about him on an Internet dating site." (statewatch)
And yes, ISPs are policemen. Not such a bad thing as long as the courts are involved and criminal procedure respected, which is not always the case.
"Danish High Court Says ISPs Must Be Internet Policemen; Have To Block The Pirate Bay" (26 November 2008)