It is a part of the jury's power, but more as a side-effect of other decisions.
The criminal justice system _does not think_ jury nullification is a legitimate use of the powers of a juror. Rather, the criminal justice system more views it as something of an unfortunate trade-off.
Juries (And indirectly, the Supreme Court) are the only part of the criminal justice system in the US Constitution. Juries are the core of the system. Everything else is minor details.
The Constitution and centuries of common law tradition thinks it is a legitimate use of the powers of a juror. Since those are the ultimate source of law in this country, the criminal justice system is wrong.
The criminal justice system _does not think_ jury nullification is a legitimate use of the powers of a juror. Rather, the criminal justice system more views it as something of an unfortunate trade-off.