Well, in the Hama flashbacks at least we see female Southern Tribe benders fighting -- so it may be a bit of a watered-down (ha) version of the gender-role divisions from the parent Northern Tribe; they've loosened up enough to let women waterbend and fight as well as heal, but the non-bending warriors are all male. And even back in the Earth Kingdom, we've seen a few female benders fighting on the coal rig, but those were for all we know just civilian villagers like Haru's family -- except for the non-bending Kiyoshi warriors, the only times we've seen Earth Kingdom uniformed troops, bending or non-bending, they've all been male. So this may just be another little touch of them showing cultural differences between the countries, or even within far-flung parts of the same country...

As for Gran-Gran Kanna, well, the Southern Tribe was definitely a lot less socially rigid in some ways -- the biggest one being the lack of arranged marriages, and that's what she ran off to get away from in the first place.