Because LJ is a dolt again:
- Pingbacks means that if you have pingbacks turned on and user X has pingbacks turned on, once user X posts a public link to your entry, you get an automatic comment to this entry telling you they're done it. Nothing more. A poor man's Google Alert for your LJ's URL.
- Yes, the placement of the FB/Twitter crossposting tickyboxes sucks. I've said.
Here's a temporary cure.