Hmm, it's strange grey area. Sometimes piracy is the only way to make the book not disappear. There are niche, low circulation books and magazines which without piracy would disappear and became almost unavailable.

Sometimes the book is no longer in the print because of many reasons:

1. Author changed her / his mind and no longer wishes to publish it, at least in the original edition / version.
2. Copyrights are being taken over and the final copyright owner ceases to republish it even when paid.
3. Copyrights owner doesn't know that his the owner of some books and it leads to the legal limbo.
4. Low circulated books & magazines don't survive until the copyrights expire - owners of the books die and their next heirs believe the books / magazines are just garbage and burn it or throw it away.

Ethics & piracy is pretty strange combination and there is no easy answer for it