Black Isle raiders were people who simple chose to make a living in the wastes by stealing what others made: Food, slaves, the lot. It made perfect sense and you could join them and everything.
Bethesda raiders are presumably cannibals, wear spiky, ineffective armour for no good reason and attack anyone on sight, to kill, I can only assume for food.
Fortunately, this is not actually hardwritten behaviour in the engine. It'd be plenty easy to make raiders etc by default neutral, unless it's a random 'raiders attack you for your stuff!' kind of event, which are acceptable. As long as it's not every raider group in the world.