Do they not also make posts on Indeed or other non-social sites?

The difference is that the recruiters come to you on LinkedIn. This is quite handy when you're currently employed since opportunities come to you that you wouldn't have otherwise looked for.

Cool, even more reason to dislike it. I want my people doing their work, not wondering if the grass is greener somewhere else.

Your personal opinion does not (and should not) dictate how others behave.