You're absolutely right, but what you're saying suggests an even more limited ability of congress. The whole point of the United State's political design was to strictly limit the ability of congress except to what was allowed. For instance the right to free speech does not grant citizens the right to free speech -- that is already inherent. It prohibits the government from being able to take that inherent right away.
The constitution is not a guideline to unlimited powers, but constraints to limited powers. If congress is passing various copyright laws for reasons other than to "promote the progress of science and useful arts" then it would be unconstitutional. In other words passing laws to enhance profits rather than to ensure creator motivation would be unconstitutional, yet that's unambiguously the motivation for nearly all modern copyright law. Nobody is deterred from creating a work because they would only own the rights to it for e.g. 30 years, let alone 110 years.
That's a good point, although unfortunately I could see them getting away with extra power due to the interstate commerce clause that they use to justify so much else.
The constitution is not a guideline to unlimited powers, but constraints to limited powers. If congress is passing various copyright laws for reasons other than to "promote the progress of science and useful arts" then it would be unconstitutional. In other words passing laws to enhance profits rather than to ensure creator motivation would be unconstitutional, yet that's unambiguously the motivation for nearly all modern copyright law. Nobody is deterred from creating a work because they would only own the rights to it for e.g. 30 years, let alone 110 years.