And the standard for rendering a JNOV is high enough that it pretty much never happens, and when it does, it happens in civil suits. Basically, if the jury came to a decision, based on the evidence, that a reasonable person could come to in the capacity of a trier of fact, their verdict will not be disturbed. When you factor in the reasonable doubt standard that the prosecution already has to overcome, it becomes clear that a reasonable jury could find someone not guilty in pretty much any criminal case.
Even in civil suits, where the standard of proof is a mere preponderance, JNOV is used very sparingly, and is usually accompanied by a new trial, not a simple reversal. The odds of a JNOV being granted in that second trial, if a second jury comes to the same conclusion, is so small as to be negligible.
In criminal cases in the U.S., only the defendant (and not the prosecution) may move for a J.N.O.V.