I found out the answer to this just yesterday in a different context. Here's how it was explained to me:

When those who opposed the impending passage of the DMCA realized they couldn't defeat it, they (mainly EFF at that point) decided to salvage what they could, which is to stick in a clause to allow the Librarian to decide exemptions. The *AA didn't try to shut that clause down because they thought it was basically a joke and would never amount to anything. But in reality the Librarian has indeed exercised some power, so it's considered a minor win for consumer advocates.