I believe there's some confusion here between legal and moral obligations. There's no legal obligation for the jury to find in any direction, yes - that is the basis for nullification. However, whether nullification is an intentional feature, or an unavoidable but undesirable side effect of the jury trial system, is very much up for debate. If you look at juror's oaths, they generally tend to embrace the latter approach. For example:

"You must not substitute or follow your own notion or opinion as to what the law is or ought to be. It is your duty to apply the law as I explain it to you, regardless of the consequences"

So when a juror nullifies, they break that oath. Insofar as that oath exists, it sets society's expectations of what the jury does.

"Impartial" was perhaps not the best choice of word for this, I agree.