[tylervigen]

If a defendant has the constitutional right to trial by a jury, and that jury has autonomy to make an independent decision, then jury nullification is a possible outcome.

If jury nullification is not a possible outcome, then either the defendant doesn't have a right to trial by jury, or that jury is not allowed to make an independent decision.

Defendants don't have a direct constitutional right to jury nullification (the Constitution doesn't say anything about nullification). It's just a logical consequence: if the jury really can make independent decisions, then nullification is necessarily one of those possible decisions.