Tatooine was pretty well known for it's crime syndicates, I wouldn't be surprised if the empire just kept a particular crime lord (Jabba for example) in power because the "the devil you know" situation. All the troopers on Tatooine I believe are part of Vader's elite unit (Same guys that stormed the rebel ship in the beginning.) That being said, I would not be surprised if they left a fairly big contingent of troopers there after they had landed, they just probably would not receive reinforcements.