That's certainly part of it, though the "ham sandwich" saying is really more about the fact that, since the prosecutor has effective control over the entire grand jury proceeding (which witnesses they hear from, what evidence they see, etc.), a prosecutor can pretty much get a grand jury to do he wants it to do. If a prosecutor wants an indictment, he'll get one (usually). If a prosecutor doesn't want an indictment, the grand jury will return no true bill (usually). Going through the grand jury evidence in the Wilson case, it is pretty clear that the latter is what happened - the prosecutor did not believe there was a case against Wilson (because there was not a case against Wilson), but was essentially forced by public pressure to present the case to the grand jury, so he presented a case that he knew couldn't possibly result in an indictment.
And so did the grand jury.
The smartest thing the prosecutor did in the Wilson case was release the evidence that the grand jury saw the same day it was presented. Transparency. I personally thought it was a good thing, others disagreed. At the very least the truth was out there for anyone who cared to see it, clearly these black based organizations demanding another grand jury didn't look and frankly don't care what the truth is.
They just want a white cop in jail. Period.
Note that what we are told of the letter from the Inc. Fund does not address the strength of the forensic evidence, which is quite telltale as to the purpose of the letter.