I think the reason people usually gain some fat when they're bulking is because they eat enough carbs and fat and protein to make absolutely sure that they've got enough for their muscles to grow. It's better for your body to have a surplus than a deficit....
.... also, when bulking people tend to eat sources of protein which have a lot of fat, like whole eggs and nuts.
You don't need to gain much fat at all, I don't think. But when people aren't too worried about it, they're not worried about junk food either. If you eat clean, but just a LOT of it, I don't think you'll gain too much fat.