[isanengineer]

I didn't understand that either, but I found a good explanation on page 2:

"The fair use question is a mixed question of fact and law. Reviewing courts should appropriately defer to the jury’s findings of underlying facts, but the ultimate question whether those facts amount to a fair use is a legal question for judges to decide de novo. This approach does not violate the Seventh Amendment’s prohibition on courts reexamining facts tried by a jury, because the ultimate question here is one of law, not fact. The “right of trial by jury” does not include the right to have a jury resolve a fair use defense."