Briefly, the Constitution says that Congress can grant copyrights for a limited time, to encourage the artists. However, current copyright law grants copyrights that last for 70 years after the author's death. Clearly, no amount of encouragement will bring more from a dead artist; we will never see more works by Walt Disney, John Lennon, Robert Frost, or Jim Henson. This case, soon to go before the Supreme Court, will hopefully overturn the most recent copyright extension act, and finally release Mickey Mouse (and thousands of other works) into the public domain.
(Every time Mickey Mouse has nearly entered the public domain, extensive lobbying by The Walt Disney Company and friends has resulted in a copyright extension act being passed. This is particularly annoying since so many of their works are based on stories in the public domain!)