1). Tusken Raiders are using low-quality salvage rifles, and trying to aim out of a mask that would functionally be difficult to aim at anything in. Plus, they are established as being fairly primitive, so they most likely have no training beyond knowing which end shoots 2). Stormtroopers are using a helmet that, from practical experience, sucks for accurate aiming, due to the design. Also, the E-11 is essentially a storm weapon, designed to saturate an area, rather than hit with any precision. Also, they're more trained in squad-based or mass-fire tactics, using their superior numbers to overwhelm targets in a barrage of firepower.