These guys did get jobbed by the system.
First, criminal charges weren't pressed because of the requirement that an attorney be appointed for their defense in criminal court, and then a swift summary judgment obtained without trial in civil court.
Examining the evidence, makes it even more murky with basically none of the news footage lining up with what the DA was claiming in terms of racial epitaphs, etc. One of the men interviewed after the event, doesn't mention anything other than their attire intimidating voters as the reason for their removal from the site. Even the police failed to arrest them on that occassion, merely releasing them at a distance from the site after picking them up. One of the two men in question had been retained as a poll worker and lived in the building where the votes were being taken. Their actions clearly weren't part of some larger black panther attempt to alter the vote, probably they were just trying to express their happiness that a black person was on the ticket in a misguided way.
Unfortunately, to me this seems on the balance like a referendom on attire at the polls. I'm not sure why or how it's become such a cause celebre of the right-wing. Holder is exactly right to have dismissed the charges the way he did.