I think it’s because when you really look at it, Star Wars and WH40K are the same story from opposite perspectives.
One side sides with a collectivist cosmic force that pulls all living beings in the cosmos, or at least all that are attuned to it, together in a psychic unity, that has its own will, but doesn’t require all people to attune themselves to that will.
The other side is an empire of man, focused on bending that same force to their will, and on asserting the will of man as the dominant force in the universe.
In Star Wars, the Force, as it’s simply called, is blindly trusted because it’s just treated as the essence of life itself. People who want to bend it to their will are essentially characterized as abusers in an abstract sense, because they bend and break this ‘essence of life’ to benefit their own lives.
In WH40K, the Immaterium is not life, but mind, and it is biased towards the darkness, the nightmares, because when you mix the bad and the good indiscriminately, the bad tends to destabilize the good. To follow the Warp would be the macrocosm of giving in to your every irrational whim. It makes sense to want to dominate that which is made up of thoughts and emotions, because in real life people are expected to control their emotions and get ahold of their thoughts.
Nonetheless, the commonality is basically good enough to turn Redditors evil because big empire, big man is the kind of ‘power’ that they love to fight for the sake of doing so, and not really for the sake of justice. They do it because the underdog is always right according to the cheap media they wrongfully use as credo rather than potato-chip entertainment. If there’s no boundary between their real beliefs and their fictional consumption, why would there be one between one fiction and another? Their adherence to their principles would almost be admirable if their principles weren’t so fucking dumb, and themselves formed by them lacking principles to the point where they listen to, and thus become, screaming imbeciles online that demand that everyone listen to them unconditionally as they spread their progressivist dogshit dogmas.