The most commonly accepted definition of right(n) is "a just claim." And in the context of the law, "just" = "legal."
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My point exactly. If it were illegal to nulify jurors could be charged for doing it. They have a just claim to the power, it is legal, it is a right.
Jurors can't be charged with this because there is no way to prove it. And the jurors themselves wouldn't be prosecuted, by the way; it's been common-law doctrine for centuries that jurors are not to be prosecuted for anything they do in the rightful capacity as jurors. If a jury were found to have engaged in nullification, the result would be the overturning of their verdict as null. And even that would carry its own slew of double jeopardy issues.
Jurors are called upon to make decisions. Trying to prove that they made a given decision by using a certain rationale is tantamount to attempting to prove a thoughtcrime. This isn't your average state-of-mind issue, either: the result that a jury reached is one that they can legitimately reach. Juries are not held to their results. This isn't like negligence or premeditation. You actually have an objectively viewable red flag in criminal or tort cases that use those concepts.
A prohibition on jury nullification would be impossible to enforce and so can't really be made illegal. It is therefore recognized as a power, albeit one to be exercised under the radar. But it has no legal sanction whatsoever, and therefore cannot be recognized as a right.