These guys simply don't quit. They are either really persistent and don't mind earning their share from time to time with these things or everyone is lying when they say their demands are not met and they don't pay them.
I wonder what's really their motivation...
If I had to guess, I'd say they probably did get paid a few times, otherwise they wouldn't bother. This post confirms that, but it may as well be a lie:
From your trust feedback, I believe you have been doing this kind of extortion for a few months already. Have you ever got anything from it? Why don't you use your skills and knowledge on more positive stuff?
If I didn't get anything, I would stop doing this long time ago. Most admins simply pay, without complaining publicly...