I don't know enough details to actually judge the judge one way or the other, but in theory it would be entirely appropriate for a judge to say this if the first amendment was not relevant to the defendant's guilt or innocence and bringing up the First Amendment would only confuse the jury.

I am not a lawyer, but if the judge was wrong that the First Amendment is not relevant to the crime and the law Olson is being charged under, then the defendant should be able to make that case in appeal (i.e. that the defendant was guilty of the crime, but that it is unconstitutional for this to be a crime).

It is someone of a legal philosophy debate: is the role of a jury simply to return a verdict of guilty or not guilty, or does the jury have the right to decide on the law itself?

In modern legal practice in the US, the law itself is not argued before the jury. What the judge seems to be doing here seems to be standard legal practice.