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. If it was still dark out, then she did a dumb thing in service of the plot.

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.

I've probably just given the whole kidnap scenario more thought than George did! ;)