This definitely seems like something that one should be able to enable or disable on a per-journal basis, yeah. If you don't want people to be able to crosspost comments they make in your journal, then they shouldn't be able to. Of course, people can and will manually crosspost anything they want, but that doesn't mean we should give them tools that make it easy.

I'm pretty public on the web, but I'm extraordinarily lucky that I'm able to be that way (for instance, I don't have emotionally abusive parents, or a job that requires me to censor myself online much), and I understand that not everyone can be or wishes to be so public.