In most jurisdictions, copyright infringement isn't a crime, it is a civil tort, meaning you can be sued by the copyright holder for damages. Most big media copyright holders (MPAA/RIAA/etc) have a policy of only going after uploaders because it just isn't cost effective to go after downloaders for damages. A downloader can only be sued for the retail value of what they downloaded, while uploaders can be sued for the retail value of what they uploaded times the number of downloads. But this isn't codified in law, it is just corporate policy.