They’re also likely doing it to the monkeys because the consequences are lighter than doing it to humans (not that that makes it more or less ok). Note that they want to see them in diapers and bottle-fed, which is not something monkeys typically do. Specifically, I suspect that while the acts are likely illegal, possessing the media itself probably isn’t, while it very much would be if it was a human child.
I used to work on a unit for men with charges, so we often got men who could not be placed on the coed unit next door. One time when we had a particularly high number of men on suicide watch, a member of administration came to investigate why we had so many. I informed him that many of them were due to sexual misconduct; they would engage in self-injurious behavior so that they would have 1:1 observation assigned, which is a task mostly done by nursing assistants. Because Healthcare is largely female dominated and nursing assistant is often a role for less experienced workers or those in school, most of the nursing assistants were female and in their early 20s (if not 18-19). They then had a young woman to masturbate to that would literally be fired if they lost line of sight.
These observations were also usually ordered by a male doctor who wanted to play it safe rather than risk his medical license on one of them being willing to actually off themselves when he could have ordered the observation. I typically managed this by making sure to assign a male staff member, (and in particular the gassy one when possible since he preferred sitting to rounding anyway), but I wasn’t always on shift to make those assignments and often even when I was there were only female staff members on duty and the best I could do was assigning the bitchiest middle aged woman possible, but I digress.
The admin guy wanted to know if they were somehow misguided enough that they thought this would be sexually appealing because some fucking how the (as it turned out) director of psychiatry for the hospital didn’t have the perspective to understand that that was just the closest they could get to regularly raping someone without consequences (I will give him the grace of understanding that he was not and likely never was a forensic psychiatrist).
TLDR; what they can “get away with” is definitely a factor here. This isn’t them getting off on the most extreme thing possible. They would do worse / more illicit if they could.