This cuts both ways as judges often adjust their sentencing downward based on mitigating evidence. For both aggravating and mitigating circumstances evidence does need to be submitted, and there are standards of proof to be applied. It's just that the procedural rules can be different and, depending on the context and jurisdiction, sufficiency can be decided by the judge alone. In some jurisdictions, for example, aggravating evidence may need to be put to the jury, while mitigating evidence need not be.

The U.S. is rather unique in providing a right to jury trials for most--in practice almost all, including misdemeanor--criminal cases. And this is a major factor for why sentencing is so harsh and prosecutions so slow in the U.S. In myriad ways the cost of criminal trials has induced the system to arrive at its current state favoring plea deals, with overlapping crimes and severe maximum penalties as cudgels. Be careful about what kind of "protections" you want to impose.