If the locals don't consider it Fire Nation territory, then it's occupied land and not an invasion or raid, and any "civilians" are part of the occupying force. They shouldn't be killed out of hand or in massive collateral damage, but if their comforts are limited because of raiding, well... That's what happens when you move into hostile territory. If you don't like it, maybe you should go back to where civilian shipping is secure.

But hey, it's not like the Fire Nation ever stooped so low as to attack purely civilian settlements and abduct and kill non-combatants just because they could. Right, Hama, Kya?