I think you got it. At some level, the word "reasonable" is an appeal to what is basically common law understanding. Forgetting the obvious question of why such a statute might be needed in the first place, it's not the use of "reasonable" ipso facto which is offensive to justice or liberty. But when a judge defines and redefines things to mean whatever he wants, then he's essentially enabled to dictate the outcome of a trial to the jury.
<chuckle>
Ever since John Lilburne's third trial for treason, when a huge cheer went up from the gallery at the jury's not guilty verdict, I guess courts have been a little more sneaky in their method of influencing the jury. In Lilburne's case, I think it was Oliver Cromwell's supreme council who later examined the jurors, pretty much all of who said they judged according to their conscience, or didn't answer much at all. One juror flat out told them the prosecution didn't prove its case, which was an artful dodge and a slap in the face because everybody and their cousin knew Lilburne had transgressed Cromwell's and his Parliament's laws by writing tracts highly critical of the government and its destructions of English liberties.
Several judges tried it again later with William Penn--directing a verdict. The jury ignored them, giving them a pain in the neck by finding Penn and his co-defendant guilty of "speaking" which wasn't a crime.* Then, the judges threatened the jury, and the jury basically said, "OK, then. Not guilty." The judges threw them in jail "...without meat, fire, or tobacco." One of jailed jurors, Bushell, petitioned a higher court for
habeas corpus. That court explicitly stated that jurors could not be punished for their verdict, ordering Bushell's release.
So, overtly directing a verdict hasn't worked much since about 1670, thus judges have gotten sneakier.
*Aaaaa--hahahahahahahahahahaha!!! I love that verdict! The judges wanta
order us to give a guilty verdict? OK. We'll turn it around and say he's "guilty" of committing a non-crime for which they cannot sentence him. Sublime finesse! Oh, delicious!