"The Congress shall have Power (1)... To promote the Progress of Science and useful Arts (2), by securing for limited Times (3) to Authors and Inventors (4) the exclusive Right (5) to their respective Writings and Discoveries (6)" (Article I, Section 8)

(1) This is a grant of power, not a mandate. Congress may exercise it, but is not compelled to do so.

(2) The goal is for the public benefit, not for private gain. It is meant to advance our knowledge and culture, not provide a permanent rent-seeking scheme.

(3) There is nothing "limited" about "life of author + 70 years"; the life of the author is technically indefinite, and furthermore 70 years is on the order of a person's life expectancy. The original term of copyright in this country was 14 years from date of publication.

(4) In no way is this protection afforded to publishers, producers, distributors, or other people whose contribution to the work is laborious but not novel. Novelty is subject to protection, not labor.

(5) Nowhere is there a requirement for violations of this "exclusive Right" to be crimes; nor is the government charged with enforcement of it. Copyright was meant to be a civil, not criminal, matter.

(6) The recording of a musical production or motion picture is not a "writing or discovery" and is thus not technically subject to these protections to begin with.

As I understand it, all of these limitations have been overridden by various treaties signed by the United States. Nevertheless, if you're going to use the Constitution as the source of your argument, it's best to know that what we have and what was intended are two drastically different things.