A pirate is a criminal at sea, who inititiates violence against sea travelers. Pirates steal property (like vessels) and valuables, and it's not uncommon that pirates murder their victims, or take them hostage for a ransom.
How the term 'pirate' ever could be used to denote kids swapping MP3's, is unfathomable to me. The analogy is ludicrous. But maybe it could be because pirate (the seafaring kind) communities in the 18th century had a liberal approach to freedom, which was unusual in that time, and maybe that extrapolates somehow to the liberal file swapping in our digital age. Which doesn't, by the way, harm anyone, nor takes away things from people.