It is unfortunate to admit, but United States grand juries are too often these days getting involved in acts of political persecution. There are separate branches of the government for a reason, and it is a sad fact that those various branches have begun to collude with one another today.
Yes, US grand juries are sometimes part of political persecution, because a grand jury will indicte anybody. There is joke among attorneys that a grand jury will indicte a ham sandwich. This is not to say that prosecutors are much better. George Zimmerman was definitely charged only for political prosecution, and that was by a prosecutor.
Because grand juries are such a vital part of the US judicial system, to say that they have never been used for political persecution would be the same as saying that the US justice system has never persecuted anyone. A simple look at the history books will show that there was a great deal of legal persecution of suspected communists within the past 60 years, and the grand jury was certainly a tool in that process.
As a rule, I would say that US grand juries are not part of political persecution. I think if a jury member ends up in this position, they more than likely put themselves there. I believe political persecution is hard to direct to jury members, but I can see that some cases may yield that result, especially if the juries decision was viewed as different.