This anti-gang law is basically a state version of RICO, the "racketeering influence and corrupt organizations" law used by the Feds to take down organized crime. According to Morris Dees of the Southern Poverty Law Center, one of the foremost domestic anti-terrorism organizations  (which is representing some of the accusers), this may well be the first time the anti-gang statute has been used in such a way – and his opinion is that this is "a very good use" of the statute.
The court case around this is likely to get fairly complex, and we can expect the members of "Respect the Flag" (the organization being prosecuted) to challenge the anti-gang statute on various constitutional grounds. Interestingly, those probably won't be simple First Amendment grounds, as terroristic threats are something explicitly not protected under the First Amendment; you can't show up and argue that a law infringes on a right when conviction under that law first requires conviction of an underlying offense which would, in turn, require that you have gone beyond that right anyway. Challenges will probably instead come around the vagueness of the statute. 
However, this law has been on the books since 1992, when it was passed for the fairly overt purpose of going after black street gangs. The reason this prosecution was noted as unusual is because it's very rare that white people get charged under this law at all, and to charge them for an attack against a black community is nearly 180° from what the normal use of this law has been.
So it's not at all clear how this is going to play out in court, but the simple fact of the prosecutor's decision to charge them in this way is significant, not least for what it bodes for the future. Apparently there has been a fairly significant rise in similar incidents ever since the Charleston massacre in June; prosecutors deciding to go after this, even in overwhelmingly white suburbs which are starting to see an increasing black population (a place where police have traditionally been very deferential to white preferences), is not trivial.
 The NYT article just mentions a "party," but other articles go in to more detail: http://www.rawstory.com/2015/10/grad-jury-indicts-15-on-gang-terrorism-charges-for-parading-confederate-flags-through-black-childs-party/ . The NYT article gave the best coverage of the legal situation, though, so I picked that one for the cover.
 "Making terroristic threats" is the legal term for making threats of violence with intend to cause terror; it has nothing to do with the modern political sense of "terrorism." The relevant law in Georgia is here: http://law.justia.com/codes/georgia/2010/title-16/chapter-11/article-2/16-11-37 , and it's pretty typical of similar laws across the country.
 http://law.justia.com/codes/georgia/2010/title-16/chapter-15/16-15-4 .
 In this case, very much in the modern sense of terrorism. The SPLC was founded to deal with organizations like the Klan and the Nazis, and today is one of the major resources for everyone from scholars to law enforcement dealing with domestic terrorist organizations. Most importantly, they are a legal organization, litigating against violent hate groups, and often take cases such as these.
 You can read more about the history of this law, and of questions about its constitutionality, here: http://maconmonitor.com/2015/05/03/why-georgias-street-gang-act-may-be-unconstitutional-as-applied-to-macons-hybrid-street-gangs/