Actually, it is the freedom of speech clause in the First Amendment that gives fair use rights, as interpreted by the courts. It could even be argued by a future court that the First Amendment invalidates copyright altogether; it wouldn't be the first time a court had changed direction like that.

That is a less extreme change than that from Plessy vs Ferguson to Brown vs the Board of Education, for example. And a good case can be made for freedom of speech trumping copyright; the Constitutional argument for copyright is a practical one for "encouraging" the advance of arts and sciences; eternal copyright has already undercut that argument.

It's not in the constitution, but it is codified in law (Copyright Act of 1976, I think). But it was common law for a long time before that.

I meant to respond sooner. I think you are correct that Fair Use originally developed through case law, (owing to the First Amendment). It was later codified. The First Amendment is part of the Constitution, and take precedence over the Articles, thus Fair Use is necessary to reconcile copyright with the First Amendment (Which is part of the Constitution).