IANAL but I have it on good authority, including direct quotes from lawyers, that the basis of copyright (and patent) in the US is to be found in Article 1, Section 8, Clause 8 which reads:

To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;

Based on historical documents that have been pointed out to me, the highlighted section is the purpose of copyright. It was felt that the government did a wrong to have such a thing as intellectual property. The government does harm by supporting a temporary monopoly. But it would cause greater harm if people were not encouraged to create intellectual discoveries. Therefore the harms are balanced by limiting the scope of the monopoly and providing exemptions for personal use.

The story that the primary purpose of intellectual property rights is to make holders of these temporary and limited monopolies happy is a myth. It is to provide them enough incentive to work for the ultimate enrichment of the public domain, a fact which Congress seems to have forgotten too often in the last century. :-(