The US Constitution gave Congress the power—but not the obligation–to enact copyright laws in pursuit of certain social goals. A mistake, IMHO, but it was certainly mentioned as something Congress could do at its discretion.
And then the First Amendment promptly nullified that power by categorically stating that the freedom of speech shall not be infringed. Distribution of content is speech, and copyright enforcement penalizes it, which infringes the freedom of speech. This is how the exceptions for Fair Use came about, by attempting to reconcile the Copyright Clause with the First Amendment, but Fair Use is only a half-measure which does not fully avoid the infringement of the freedom of speech. When you have Rule A saying that you may do something and Rule B saying that you must not do that thing, they don't combine to create a compromise rule which says that you can do that thing most of the time with a few narrow exceptions.
