Here's a long but interesting and jargon-free explanation of how the RIAA shakes down people for $3750 a pop, and why most people who get sued by the RIAA settle out of court even if they haven't ever downloaded music on a P2P network.
The person being sued may have never shared a file, or logged on to a P2P network. They haven't been convicted of any crime involving copyright protected material, nor have they been charged with one. They've simply been sued in a "civil" action. In the United States, anyone can sue anyone else for anything at any time. It's quite possible (and maybe even more likely than not) that these average people didn't violate anyone's copyright.
In any event, the burden of proof for a civil suit is much lower than that of a criminal prosecution. There is no possible way that anyone who has been sued by the RIAA could be convicted of any crime with the evidence the RIAA collects.
