Didn't the pod race course cross their territory? And the first time we see them in Star Wars, one is about to shoot at Luke's landspeeder. It's implied he's going out into a dangerous area looking for Artoo.
Luke tells Threepio it's too dangerous to go out at night because of Sand People. IIRC, Ani's mom gets grabbed in the morning, presumably when the sun is up, because nobody who's spent a lifetime living on Tatooine would be going mushroom hunting at night.
A simple kidnapping to gain a wife or slave might be in character. (Right out of old westerns.) The torture part makes no sense, other than to have Shmi croak the minute Anakin gets there, so he can go nuts right on cue. A nomadic tribe with limited sources of water can't afford to waste anything on keeping someone alive just to beat them.
"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.