"Their territory" is a tricky concept if the Sandpeople are a nomadic people. "Dangerous areas" for settlers tends to mean places they haven't managed to kill off enough of the local nomads yet. I doubt there's a sign that says "NO TUSKENS" that the Sandpeople stop at. And since AOTC is 20 years before Star Wars I think it's a possibility that there were more Tuskens and less settlers, meaning more danger.
So Shmi was picking mushrooms, normally safe, but a group of Tusken Raiders picked that morning to go raiding (it's what they do) and she was unlucky enough to be caught. It's a risk people take settling in dangerous territory. No reason to assume she was 'dumb' at all or that it had to be dark.
Since Sand People, every time we see them do anything are in a state of active agression against the settled people of Tattooine, I don't see why its unlikely they would kidnap a settler if the oppurtunity arose. If they then beat/tortured/raped her (never made clear in the film) that plays right into their normal pattern.
Since 2 of the 3 times we see Sandpeople they are taking hostages, I disagree that they must lack the resources to take hostages. Instead I would say clearly they do have the resources, because they do it.
Nomadic people can claim ownership of lands with important things like wells on them. It comes up a few times in Lawrence of Arabia.
In the only other pre-SW story Marvel ever did, (and presumably with Lucasfilm's ok on it) Tuskens went on the attack as mineral prospectors accidentally poisoned a sacred well.
Based on the movies, the Tuskens seem to be in the hills/mountains and canyons. We don't see them out on the flat open desert where Luke is out daily fixing vaporators. At least not in the daytime.
I didn't say Shmi was dumb. She did a dumb thing because Lucas required it for her to die. In that instance, Tuskens were handy monsters.
I didn't say they didn't have the resources to take hostages, just that you don't waste them on someone you intend to kill. Put them to work for you, otherwise it's a waste of water. Why do I have the sudden urge to go watch Dune? ;)