US Vigilantes Fight For And Against ISIS

Here is a report about Americans who have joined ISIS:

http://www.cnn.com/2015/03/05/us/isis-us-arrests/

And here is a report about US vigilantes who are fighting against ISIS:

http://www.nytimes.com/2015/10/04/magazine/meet-the-american-vigilantes-who-are-fighting-isis.html

Q: Aren't BOTH groups engaged in illegal activities? Why then is the US persecuting the former while apparently tolerating the latter?

A: Perhaps it's because ISIS is a threat to the US homeland.

But is this a credible threat, or just another FBI created bogeyman?