It's also the regulation, where most systems still look at "one pirate copy" = "one sale of lost profits", especially when pirates end up in court. If the book (or game or whatever) is not sold anymore in any way where you could give the copyright holder money in an easy accessible way (eg. buy it on amazon, or a local bookstore), they shouldn't be able to claim losses from piracy, since they clearly don't want your money.

On the other hand, there are grey zones here, the lord of the rings books (still copyrighted and easily obtained pretty much everywhere) have been translated into my language many decades ago, and many of us read and liked those translations, but when the movies came out, a new translator did a new translation, where they changed a lot of things, including the last names of bilbo and frodo (Bogataj->Bisagin) and the Shire (Grofija->Šajerska), and the old version is sadly available only in paper form on second hand markets. On one hand, copying that if you only want this specific version would not cause a lost sale, on the other, you can get new translations (or english originals) pretty much everywhere.