Most common regulars don't want to get banned. There are a few who do, like Baloki. If you read his LJ prior to his banning, he's been complaining about wanting to get banned. Or something. It also by the content of the post. If it was in a photo thread, it's likely a mistake. If it's in its own thread, probably done on purpose.
Just because they don't want to get banned doesn't mean Peter didn't intentionally post that image. I would say the more likely event was he thought the image was appropriate, posted it, then said it was accidental when he got in trouble for it.

Not everyone reads their PM.
Not the Staff's fault. And for every banning I've heard of, they'd been warned in threads, too.

Looking forward to Aaron Freed quoting this in four years.
x-freakin'-D