The (Il)legality of suing the file-sharing masses

If an internet connection is classified as a means of personal communication (as it justly should be), just like a phone line or the snail mail, it would be illegal for the MPIAA, the RIAA, the ISP or the po-lice to go snooping about monitoring the data that is exchanged without a warrant, which would mean all those cases they have against individual downloaders/file-sharers of copyright content would be thrown out of court. Why isn't this happening? (Yeah, yeah, the US legal system has sold out to all the corporates and all that BS, but my point still stands: isn't it a violation of the privacy act, due process and an unlawful interception if an internet connection is monitored without a warrant? That would certainly turn the tables between plaintiffs and defendants in my book.)