Festivus, this touches on both points you say it doesn't:

1)they does not violate any other part of the constitution
2)they do not grant the copyright holder exclusive rights in perpetuity

For #1, congress grants itself an uncircumscribed power because it implicitly gives itself the ability to do #2. We're actually arguing about this on a different thread:

http://www.quicktopic.com/boing/H/QfwNhUanHSWM