While the details are still unclear, it appears that LJ have enabled it in such a way that it may possible to crosspost content from other people's locked communities and journals. And they have not offered users an option to disable it on their own journals, to prevent others from crossposting content.

I'm sure there are a lot of people who use other forms of social media, but choice is important, and so is privacy. Many of us want to be able to keep our blogs and communities separate from RL social networking - for many reasons.

And then there are people like myself who don't use those services who would prefer not to have them thrust upon us. :D