Good question and that in a few words sums up the case for sttorneys of P2P sites.
actually, when you are borrowing your friend's stuff, it is not an illegal broadcast. (though unapproved sharing is also banned). When you use a P2P site you inadvertently broadcast or get intellectual property without permission.
Companies are not actually worried about you though......they are worried that if such sites are encouraged it will encourage piracy of copyrighted material