The US Constitution establishes copyright as a temporary monopoly on who gets to make copies of a published work. Why temporary? Why shouldn't an artist have that right in perpetuity? I'm no constitutional scholar, but let me suggest two reasons, one of which I'm pretty sure is what the Founding Patrimonials had in mind.

First, we grant only a temporary monopoly because we want to make sure that the fire of the public domain is kept richly stoked. So, we balance the desire to compensate artists — to be fair to them and to give them an incentive to continue creating — with the public good of having public ideas and melodies floating around freely. The existence of copyright means that we don't think creators have an unimpeachable right to their own creations.