But then you're bringing opinion into the law. If the jurors on one trial think that 30% isn't fair, but jurors on another trial think that it is fair, you haven't got a fair system anymore. Isn't the law based on justice and fairness?

And this doesn't only apply to tax laws. There could be a difference of opinion on other cases too.

While the law nullification idea is a good idea in theory, it starts to break down in practice.

The whole idea of having jurors is to bring opinion of peers into making judgement about the case.

Of course it's better to make a system that would be consistently fair (e.g. keep taxes at the level vast majority of society supports it). But if two available options are "not fair" and "not always fair" - I'd pick "not always fair".

The supreme court has in past found that punishments which are capriciously applied are inherently cruel and unusual (Furman v. Georgia).