This is part of the trap set by the Emperor if Leto tries to run Arrakis as a sub-fief the war will inevitably reach Caladan. And at the beginning of the book the Atreides do not have the resources to hold both planet. They have a small very skilled professional officers corps that can punch above their weight but very little foot soldiers and that where the Fremen come in. Most of the major players except the Harkonnen understand what the Fremen represent. The Emperor hope to destroy House Atreides and get the Fremen to stick their neck out long enough for the axe to fell. House Atreides think they can recruit the Fremen and hold on to Arrakis. The Spacing guild want spice to keep flowing no matter who mining it amazing that they let the Emperor mess with the existing power structure the way they did. And I have no clue what the Bene Gesserit want at that point they are so freaking bipolar. The only side that is screwed no matter who win are the Harkonnen. In the long run the Emperor would have double crossed them in favour of a less dangerous minor house or even the Fremen.
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