In the US, juries decide facts, judges interpret the law

In the US, juries decide facts, judges interpret the law

Note also that it's clear that Judge Alsup is conducting this trial with a view to a possible appeal.

By getting the jury to determine facts that may become irrelevant depending on how he decides the law, he's set up to avoid having to reconvene a jury trial with a new jury in the event that the appeals court tells him he made a mistake in his interpretation of the law.

From the outside, it looks like Judge Alsup is very aware of how important any precedent set as a result of this case will be, and doesn't want questions of fact distracting the appeals courts from the questions of law.

As a nice side effect (and I think he's considered this, too) it ensures that when the appeals court is asked to rule on matters of law, it's not completely abstract; the appeals court will be able to look at their decision in the light of a real jury's findings, and will be aware of the full implications of their decision - there won't be any "well, this is how the world should be, because I expect a jury would obviously see this case as fair use" in the appeals world.