Because these are PRIVATE calls via the switched packet network (just like a voice call, it's digitized too), so therefore you have to WIRETAP someone to figure out FOR SURE WITHOUT A DOUBT that they are downloading a copyrighted song.
You can go to JAIL for wiretapping, THAT'S ILLEGAL! so when the RIAA tells you to stop, you ask them to put it in writing and say how they found out you were transfering whatever they thought you were.
IP addresses are no different then a phone number, GET A CLUE PEOPLE! WAKE UP!
The first time a RIAA lawyer goes to jail for wire tapping, this will stop real fast.
THEY ARE THE CRIMINALS!