Or, we could think of it like this: What is trans porn typically labelled like BY cis people? Those offensive terms listed. So, why would trans women post under those terms online? Simple: Because that's what people are looking for. That's HOW people are looking for it. So, you label it in a way that gets the most clicks, even if it's directly offensive to you as a creator. Making it a sadly, self-perpetuating cycle.
This isn't just like, theorizing, by the way, I know trans women who work in porn who have directly stated that the reason they 'identify' using those offensive terms is because it's what's expected of them. Dozens of them. I've never seen one that's like, 'yeah, I'm fine with it and it doesn't feel bad' it's always, 'it feels bad, but I need the views'.
As for 'trans women using femboy', that could easily be some form of non-binary, but that term isn't inherently offensive like 'shemale' is. That one is JUST offensive.