Stockholm Syndrome wouldn't really apply here since the Krieg weren't taken from a good planet and then (somehow) end up loving the Empire after it forced them to live on a lousy one. Even if that were the case, Stockholm Syndrome is by no means universal. There are people who hate their captors on the first day and go on hating them every day afterwards. Anyone who believes otherwise needs to read the biographies of the post-war "nazi-hunters" who survived the holocaust. They spent their lives tracking down and prosecuting every last guard and clerk even peripherally associated with their captivity.



So why would real-life Corpsmen desert at the sight of beautiful world? Indirectly, you answered your own rhetorical question. That's not sarcasm or "snarkiness", either. The people on Krieg spend their whole lives on a miserable planet and they've made a religion out of suffering for Da Emprah. Sending them to other miserable, even worse, worlds only reinforces their world-view and their religious beliefs. It's a long-recognized truth that suffering tends to strengthen religious sentiment and pleasure weakens it.



This is one of the root causes why fundamentalist religious systems try to eradicate all forms of hedonism as "sinful" while using "penance" to reinforce feelings of virtue. Once people truly begin enjoying life and enjoying themselves, they're not going to give damn about what some sour zealot tells them about sin.



Fundamentalists in America ban alcohol consumption (and about a thousand other things) for the same reason fundamentalists in the Middle East ban nearly an identical list (with a few hundred more things added for good measure). Getting drunk makes people loosen up, have fun, get horny, and so on. After a few days like that, nobody's going to listen to some hate-filled old man with nothing in his life except using his holy book as a justification for telling other people what to do. That's why fundamentalists always try to destroy all sources of "temptation". If they don't, a positive source of enjoyment will always win out over enforced self-denial.



Look at SlaaneshG's artwork again. Now imagine that first breath a Corpsman takes without his respirator mask.



Clean air, the pleasant scent of what look like cherry blossoms. The wind is warm and caresses his face and neck. The Corpsman takes off his helmet, opens up the buttons on his heavy stifling coat. For the first time in his life he's truly alive and enjoying it. He won't like having to suit back up when he hears the commissar coming. Neither will the mates in his squad. In fact, the commissar just might catch a bullet one night (entirely plausible even in the GW world -Catachan jungle fighters are known for routinely killing their commissars). Or, just as likely, the commissar will wake up the next morning only to find half the men have "vanished", along with their rifles, packs, and a month's worth of rations.