i think a lot of the stuff with TS girls is to do with Prep. I honestly think a lot of them take it. It's easy to get and affordable. Some of them know each other as well, so I'm sure they'll pass on info to each other.
Of course, that doesn't protect you against all STIs but, it's easy to forget about the other STIs since nobody really talks about them.
Just because we may seem them do bareback on their videos, we can't assume they're totally reckless either. They may just be on prep.