If someone writes a poem or essay or story, and you publish it and pass it off as your own--either overtly or by implication--that is plagiarism. On the Web, if you include the author's name, a few lines, and a link, that is within the law. But if you leave off the author's name or include more than a few lines (or both), that is theft. It is a violation of the author's copyright, unless the author has sold you her rights, or has given you permission to publish the piece. Yet this is done all of the time. Just as people steal copyrighted art from the Web all of the time.
Taking someone's art or writing or music and publishing it without permission is no different from going to someone's house and stealing his stereo. The stereo doesn't belong to you. If you take it, you are a thief. Taking someone's intellectual property is also a crime.
Any time someone produces a piece of art, music, or writing, she automatically owns the copyright on it. It is not necessary to register the copyright with the government, or to put a copyright symbol on it. Registration allows you more rights if you go to court to protect your copyight, but simple ownership gives you the basic rights you need in a court of law.
I am amazed at how cavalier people are about commiting the crime of intellectual property theft. Some people even defend such theft because they do not think that artists, writers, and musicians deserve to own their own work. That means that the people who are complaining do not deserve to own their own cars, and--using their reasoning--I can go to their houses and steal their vehicles because "they belong to everyone."
And people wonder why their children have no moral values.