Why can't grand juries do it? I thought that it's when juries intentionally rule against what the law says, while saying otherwise, as a consequence of juries not being able to be penalized for their ruling. I don't seen any reason that the same can't happen with a grand jury.
A web search comes up with articles saying that it's more common with non-grand juries, but that's not the same as "cannot do it", while they also speak about a prosecutor's ability to re-indict, but that still doesn't guarantee them a charge.
Nullification is based on a contradiction in terms: juries cannot be punished for the judgments they render, and prosecutors may not re-try a case to obtain their preferred outcome (“double jeopardy”).
With a grand jury, the second term doesn’t apply: the grand jury’s decision doesn’t prevent a second indictment.
So in other words: grand juries can choose to render “faithless” decisions, but they can’t nullify a case in the sense that matters.
It may have to do with the state of accusations. Pre-grand jury there is no evidence of a crime. Once a grand jury rules with the prosecutor a crime has been defined.