For legal reasons, I would imagine. If downloading/uploading certain stuff is illegal, then they need to be able to protect themselves when it comes to tracking down the culprit. If it can be traced to the ISP, then they want to be able to prove beyond a reasonable doubt that it wasn't "them", but it was "you".
That's certainly
part of the reason, anyway. The cynic in me says the are probably also building a database of browsing habits that they can sell to marketing agencies as well.
Cheers
Mick