Over at Strategic Sorcery (On Pirates) in the comment section, Pallas Renatus wrote "Why on earth would [pirates] risk pissing off magicians of all people? This just seems like common sense to me."
Here is my theory: The pirates do not believe that the magicians can do anything to hurt them.
Heck, the pirates do not believe that the law can touch them. If the mass forces of the law enforcement agencies and the legal system cannot touch them, how can the puny forces mustered by a lowly magician affect them?
Furthermore, they argue that they are not doing anything wrong. In fact, pirates believe that writers and artists should be happy creating new material for them to use and enjoy while not recieving a single penny to offset the time and energy they used to create the work. Even if they believes in, oh say something like, Karma or the Three-fold Law of Return, they would think that it is the creators of the material that are the moral violators and not them.
And most of these pirates have no stake in the outcome of this situation, either as creators or users. They do not create anything that can earn them a thin dime (my proof: everyone I know with a monetary stake in a copyright or intellectual property is a supporter of copyright and patent laws, even if we cannot agree how strong the laws should be). And they are armchair occultists---they are not actually using the material that they are stealing.
It is this last part that makes it especially difficult for magicians to fight back (with a couple of notable exceptations). In previous ages, magicians used to blind their work to prevent it from working correctly in the hands of those people not in their own lineage. Think: Large Booby Trap, often complete with drooling demons to feast on one's scattered remains. The defense that the lineaged working magician were traditionally taught does not work if the person just downloads a pdf and never actually casts a single spell from the work (in some cases, I am not even sure that they actually are reading the works in question).
But note that working magicians are a crafty lot, and there are some interesting magical procedures starting to be developed to cope with this new form of piracy. It is only a matter of time before the pirates realize that magicians are the last people you want to rob, and that we have rolled out a whole new line of customized curses to deal with the likes of them (curses that cannot be removed with anti-virus software).