Piracy is one of the oldest crimes in the book. Throughout recorded history, a special opprobrium has been reserved for those who attack ships on the high seas for private gain: they have been regarded as hostes humanis generis, the common enemies of all humankind.

It’s easy to see why. The oceans are a dangerous place at the best of times, and all civilised countries have recognised a duty to assist those in trouble there (which is why the ­Tampa affair was such a shock to the world’s conscience). To prey on seafarers instead is rightly regarded as an abominable crime.

But just as piracy is returning to our consciousness with the events off the coast of Somalia, quite different groups of people are being tagged with the same label, such as the four Swedes convicted last week in the “Pirate Bay” trial for conspiracy to violate the copyright of various music and video distributors.

As a piece of semantic subterfuge, this is a feat probably without equal. Real pirates use violence in a dangerous environment to steal property belonging to innocent and helpless victims. Video, music or software “pirates” do none of these things. How on earth did the recording industry manage to sell the idea that there is some analogy between them?

Copyright, and intellectual “property” in general, is a matter of social convenience: it amounts to granting a limited monopoly to certain producers in return for the presumed social utility of their product. We can argue about how necessary that is, but on no account do such statutory rights amount to “property” except by a brazen legal fiction.

Genuine property rights are motivated by scarcity: we cannot all use the same land or car or saucepan at the same time, so we have property rights to allocate them to particular people. Similarly, if I take your particular copy of a book or movie, I will be rightly liable for theft. But I, and millions of others, can read or watch different copies at the same time, without taking anything away from anyone.

It’s true that the value of what you have may be reduced if someone else distributes copies of it. But that’s competition: equally, the value of your shop will be diminished if I open a rival shop across the road from you. It may be socially necessary to regulate that competition in some way, but that doesn’t make it theft.

Some of those who object to the excesses of intellectual “property”, like the Swedish operators, have embraced the tag of “pirate” — motivated in part no doubt by the romanticised image of pirates expressed in things such as International Talk Like a Pirate Day (19 September).

The recent bloodshed in the Indian Ocean might tarnish some of that glamour. But it should also remind us that the file sharers belong in a very different legal category, and a much less inflammatory one.