Something worth thinking about here is that the BOP (Federal Bureau of Prisons, which is part of the Justice Department) has long had policies in place, often imposed on them, stating that they should use their discretion in placing transgender women in women's prisons when it's necessary for their health and safety.

The BOP has a long and stubborn history of dragging their feet and almost always refusing to do that. The percentage of transgender women they've ever placed in women's prisons extremely small... already. Because they have traditionally been so opposed to doing that, we can suppose (because it's obvious) that trans advocates are correct when they say that this new policy is going to put the most vulnerable in extreme danger.

On the other side of the issue, the BOP has not tolerated any (real or perceived) risk of potential violence from transgender women in their custody. They ALREADY transfer transgender women inmates to men's prisons at the drop of a pin, responding to any reports of violence, credible or not—and that population has always been tiny anyway.

This move by the Trump administration is simple cruelty, and in response to no threat or risk.

The cruelty is the point. A certain (quite large) subset of American conservatives are rubbing their hands together in glee over the cruelty, motivated by their hatred of trans people.