Specifically all forms of intellectual property in the USA trace back to Article I Section 8, Clause 8 of the Constitution. Which gives Congress the power, "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".
Technically life + 70 years - or 1 million years for that matter - is "limited" - but I imagine 14+14 is probably closer to what they had in mind.