Update: I should've known the answer was on the internets... good thing Al Gore invented it. It turns out it's all about the Stormtrooper Effect:
The Stormtrooper effect, also called Stormtrooper syndrome, is a cliché phenomenon in works of fiction where minor characters (cannon fodder) are unrealistically ineffective in combat against more important characters (almost always the protagonists "equipped" with character shields). The name originated with the armed Imperial Stormtroopers in the original Star Wars trilogy, who, despite their considerable advantages of close range, overwhelming numbers, full armour, military-grade firepower, and noticeable combat effectiveness against non-speaking characters, were incapable of seriously harming the protagonists. The effect is generally employed either to increase the dramatic tension of a chase scene or to accentuate the heroes' fighting prowess. Many claim that Stormtroopers are, in fact, good shots, by virtue missing by only feet with little preparation time, and often firing from the hip.
Stormtrooper effect nullification in movies
In A New Hope, Obi-Wan explains to Luke that Sand People were not responsible for the deaths of the Jawas but instead makes a interesting note on the Stormtrooper's accuracy; "These Blast points... too accurate for Sand People, only Imperial Storm Troopers are so precise." Proponents have claimed this shows stormtroopers are accurate when they have to be. When Luke and his companions are escaping the Death Star later in the movie the Stormtroopers fail to hit them despite numerous opportunities, but it is subsequently revealed that they were ordered to allow the escape so that the escapees could be tracked.
This explanation seems somewhat mitigated in other cases however, such as in Return of the Jedi on Endor in which the stormtroopers very frequently and almost invariably miss Han Solo and Leia (though a blaster bolt grazes Leia’s shoulder) while the protagonists once again take down numerous stormtroopers without getting killed. Here, the Emperor explicitly states that the forest moon of Endor was to be a “trap” with a legion of his “best troops” awaiting them, apparently suggesting that he had no intention of letting the rebels escape this time.
So that explains it!