Routinely, in discussions about unauthorized reproduction, someone will get huffy about using "pirate" to describe a copyright violator instead of an armed robber at sea.
But book piracy is as old as novels. Don Quixote had an unauthorized sequel out before Cervantes' own (like Harry Potter much later.) And the term "pirate" in this sense is as old. The OED says:
A person or company who reproduces or uses the work of another (as a book, recording, computer program, etc.) without authority and esp. in contravention of patent or copyright; a plagiarist. Also: a thing reproduced or used in this way.
[1603 T. DEKKER Wonderfull Yeare sig. A4, Banish these Word-pirates (you sacred mistresses of learning) into the gulfe of Barbarisme.] 1668 J. HANCOCK Brooks' String of Pearls (Notice at end), Some dishonest Booksellers, called Land-Pirats, who make it their practise to steal Impressions of other mens Copies. 1703 D. DEFOE True-born Englishman in True Collect. I. Explan. Pref. sig. B3v, Its being Printed again and again, by Pyrates. 1758 D. GARRICK Let. 4 Dec. (1963) I. 295 But pray what performances have we Exhibited for literary Pirates, which we have rejected from the Original Proprietors? 1822 BYRON Let. 13 Apr. (1979) IX. 142 If you publish the latter in a very cheap edition so as to baffle the pirates by a low price--you will find that it will do.
I'll accept this usage of "pirate."
Some cavillers more specifically object to "pirate" to describe someone who acquires copies known to be unauthorized (e.g., by downloading them) as opposed to those distributing them (e.g., by making them available for download.) But anyone using torrents or most other P2P software is implicitly both. I'm not sure how useful it is to draw a distinction.