As far as the good of mankind goes, a searchable database of every published book freely available to the world would the fulfillment of the dream of the Library of Alexandria. It would be the premier wonder of the modern world. This should not trump writers' ability to make a living, but working towards hardly qualifies as evil, and in this case, it hardly qualifies as theft.
It is more akin to squating. If I come across a piece of land and, being unable to find it's owner, plant a field of corn, then I have taken something from the rightful owner. I have removed some nutrients from the soil, created some risk of erosion, but that isn't theft, it's squatting. If the owner never shows up to tell me to stop and I harvest my corn, replant the next year, harvest, plant... at some point the law stops considering me a squatter and says that I own the property. Because it is more important to society that the property be used and create wealt that the government can tax than that the rights of owner who has abandoned it be protected indefinitely.