The important factor behind copyright is that is only gives the industries permission to determine who can and can not make new copies. Essentially, the only person who is allowed to make new copies should be the artist, and those he licenses to do the same. The reason libraries are considered acceptable is because they never produce copies. They can only give away as many copies as they buy. You can even see this when libraries have online catalogs. They still don’t give away more copies than they have bought, even though they have the means too. In filesharing, new copies are produced. If I get a torrent of Skyrim online, that doesn’t mean someone else is giving me their copy of Skyrim. There are more copies in the world, not produced with the consent of the Intellectual Property owner. That would be the argument for the difference.