They crossed a symbolic fenceline erected by the military at Fort Benning (actually, this year, they erected 3 such fences), that's not there most of the year, but is erected only for the protest. So, yes, in large sense, they were arrested for speaking out, since the fence was put up simply, and as the loudspeakers at Ft. Benning that the military used kept saying, to protect the military from "partisan" influence.

So, the fences were there for speech reasons (there's no evidence they are there for security reasons), and so therefore climbing and crossing the fence, is an act of speech.

One may agree or disagree with the legality of the speech or argue that such boundaries are lawful (I don't, but one still might), and yet still be faced with the irony that this crime receives a disproportionate sentence to those who have been convicted of homicide.