They enjoy it because they are perverts. But the process is called "acclimatization" or "normalization." It's the process by which atrocity is made familiar, thus lessening the revulsion rejection of the subject and making it easier to accept psychologically.
Hollywood uses it all the time. For an excellent example, read - do not watch - what that utter bastard Brad Pitt enacts in current movie "Fury," where the name of playing an American soldier in WWII, he uses his fame and popularity to teach that American soldiers are psychotic, cowardly, murdering monsters from hell who are literally worse than Nazis.
Every trick in the book is used to get you to emotionally accept this message as "fact" or "truth" - necessity, reality, manhood, your own resistance to it played out by a character designed to embody the audience, etc.
It's the blackest evil, especially since its being done now to specifically normalize Islamic atrocities, and portray our own vets as violent, deranged psychotics.
That's why Leftists are extremely sensitive to it being used against their own. That's why they scream and attack pro-life protestors who show pictures of aborted fetuses. Because they know damn well the powerful effect those pictures have in destroying the normalization narrative they've set up for abortion.
See also “desensitization” and “defining deviancy down.”