Aah I’m so glad you asked!
First I just want to clear up for my fellow translation-dependent readers that “the South” isn’t quite what Hugo says– he says they’re from “the Midi”. It’s a totally understandable translation and gets across a lot of what’s intended, but it’s not quite the same thing– a bit like an American historical reference about “the Deep South” instead of “ The South”, in that there’s some specificity and nuance there. So really, I’m going to be talking about why the Amis are, except Legle, from The Midi.
TL, DR; The Amis being from the Midi is another contextual clue/reminder of how much political turmoil France is going through in canon era far beyond what happens to the characters we follow, and another nod to the Amis in general *not* being naive or ignorant of the danger of what they’re doing **It may also be an attempt to remind Hugo’s contemporary readers that the South, which seems to have fallen into a stereotype of being reactionary or monarchist, was also home to very devotedly republican thinkers and activists.
Like so much about Les Mis, it has its roots in the French Revolution. The Midi was the home of some of the earliest and most passionate revolutionary groups–there’s a reason the theme of the Republic was “ La Marseillaise ”, after all. Volunteers, agitators and organizers from the Midi were some of the most active, involved–and violent– in the early phases of the revolution, on the ground level if not always at the level of elected leaders. ***
And people of the Midi were also some of the most active, involved–and violent– in the counter-revolutionary movement, from the earliest days on.
And “on” very much means into the 1830s.