I really loved this story. My perspective was very similar to Albionmoonlight's--that this story is a metaphor for life in a small town. Additionally, I'd say it's a metaphor for poverty, and the difficulty of escaping the poverty cycle when everyone else in your immediate social circle treats the Outside World with suspicion and hostility, and when you're given every incentive not to reach beyond what your parents achieved.
This was especially apparent in how Zinhle's parents encourage her to get pregnant to escape the possibility of leaving town, or how at school there's pressure to flub your grades. While pretty much no one *tells* teens that it's a good idea to get pregnant or fail in school, it's interesting to me how sometimes there are unspoken social incentives to do so anyway. I was recently talking with a teen in my extended family about her post-high school plans, which she'd thought out really well. The whole time, her mother kept interjecting badly reasoned comments about why the teen's plans were no good, which I kept rebutting. Finally it came out that Mom just wasn't happy that her daughter's plans involved a career that would take her far from home.
I think this is pretty typical of the small-town mentality--there's a sense that leaving is the worst thing you can do, both because the outside world is Evil and Dangerous, and because if you leave, somehow it means you're rejecting the people you grew up with, that you're somehow "too good" for them. So achievement paradoxically becomes a wedge, even if on the surface your family says they want you to achieve. Far better to get pregnant or drop out and follow the same path in life as everyone else around you. In Jemisin's story, this idea is literalized by putting actual scary creatures outside the walls with nebulous motivations. No one is really sure what the AIs *do* with the children taken away, so their interpretation ends up being a reflection of their beliefs. And of course, that ties into the ending nicely, where the AI dude points out to Zinhle that her people are unwilling to fight for her anyway. When the day comes they value their overachievers that much, they'll be ready to rejoin the rest of the planet. Until that day, it's probable Zinhle's people are secretly relieved that the AIs are taking her off their hands, because her very existence is, in their eyes, a condemnation on their own way of life.