What is the particular crime? (Score 1)

Probably a stupid question, but...

Assuming the person in question did not rip each file from CD and then upload it, what crime are they actually committing? The files on their PC were simply downloaded from an outside source - is it the duty of the person uploading to maintain a copyright notice with each file or is it the duty of the downloader to verify the status of each file they download?

If it is the duty of the downloader to verify the copyright of every item they download, where does this stop? Do I have to perform due dilligence when I log onto /.? If not, then surely the RIAA need to pursue the individual who originally uploaded each file and prove the extent to which it was spread.