What if the persons computer was hacked and the hacker is unknowingly using their server to share files?
Damn near everyone has viruses/spyware these days.
If the RIAA won a lawsuit like that, then it would set a precedent where users would actually be liable for the damage/s their computer/s do regardless of their knowlege or participation of the malicious events.
Then sys admins could start suing people for not keeping their shit up to date and being naive enough to open unexcpeted file attachments
