It's a matter of scale. We normally operate far above the concerns of a few hundred commoners. As much as I liked the previous chapter, I don't want to spend an inordinate amount of time dealing with them.
When we first took over Torturer's Deep and began expansion, the majority of our navy personnel were former pirates, who I guarantee you were worse than the bandits living in this forest. Since then, we've integrated former slavers, sell sword companies who were little more than well organized bandits who worked for the highest bidder, Dothraki of all things, and so on. The Imperium can easily absorb 300 commoners without even blinking, even if most of them aren't great people.
And if they don't want to cooperate, it is the height of simplicity to lead Stannis' men to each of their camps using False Ravens, where they will be killed or sent to the Wall, or where ever else he deems fit. We're by ourselves right now. Killing the entire bandit camp would be the work of seconds, but rounding them up when they scatter into the surrounding forest would be an issue, and this is only one of six camps.