Juries are given specific instructions which includes something to the effect of "it is your duty to determine if the defendent violated the law, regardless of if you agree with the law". The possibility of jury nullification is not included in those instructions.

Before being selected for a jury, jururs are asked (under penalty of perjury) if they can apply the law regardless of there personal feelings about it. Anyone who seriously considers nullification would be lying if they said yes, and would likely be excluded for saying no.

At least in the US, all jury nullification is, is the fact that the jury's decision making is above review. It is not so much that jury nullification is legal, it is that any ban on nullification is unenforcable. The courts therefore do whatever they can to avoid nullification.