The Atreides are still agents of a foreign empire occupying and exploiting Arrakis. They impose the Imperial hierarchy and law on the unwilling populace, by force, if necessary, and they extract the resources. They do try to treat their subjects well within the boundaries permitted by the system, unlike Harkonnens; it's just that the system itself is inherently oppressive, so the best they can do is being "good feudals". Paul even spells it out at one point:
“You sense that Arrakis could be a paradise,” Kynes said. “Yet, as you see, the Imperium sends here only its trained hatchetmen, its seekers after the spice!”
Paul held up his thumb with its ducal signet. “Do you see this ring?”
“Yes.”
“Do you know its significance?”
Jessica turned sharply to stare at her son.
“Your father lies dead in the ruins of Arrakeen,” Kynes said. “You are technically the Duke.”
“I’m a soldier of the Imperium,” Paul said, “technically a hatchetman.”
Kynes’ face darkened. “Even with the Emperor’s Sardaukar standing over your father’s body?”
“The Sardaukar are one thing, the legal source of my authority is another,” Paul said.
“You sense that Arrakis could be a paradise,” Kynes said. “Yet, as you see, the Imperium sends here only its trained hatchetmen, its seekers after the spice!”
Paul held up his thumb with its ducal signet. “Do you see this ring?”
“Yes.”
“Do you know its significance?”
Jessica turned sharply to stare at her son.
“Your father lies dead in the ruins of Arrakeen,” Kynes said. “You are technically the Duke.”
“I’m a soldier of the Imperium,” Paul said, “technically a hatchetman.”
Kynes’ face darkened. “Even with the Emperor’s Sardaukar standing over your father’s body?”
“The Sardaukar are one thing, the legal source of my authority is another,” Paul said.