I'll just put it bluntly. Persuasion is not control. People are persuaded of their own volition, mainly by presentation of a good argument supported by evidence. They aren't subliminally "controlled". No grand jury member, randomly selected from the public, has ever pointed the finger at the prosecution and said they'd been offered an inducement to vote guilty.
Bias is not a crime. Everybody has political bias. Everybody. The notion is that we would expect that they do their job in good faith, abide by their oath and make rational and sound decisions, free of corruption. We know that's not always the case (think supreme court) but it isn't cause to point in that direction anytime you don't like the outcome of a decision or grand jury vote. To claim that because one is corrupt that they all are (at convenient moments) is risible.