In other words, they only paid a few people so it doesn't look like a complete scam. So they can say it was a mistake etc when confronted with it. When they told you to mail and then did not react to anyone that mailed, from there it was obvious what was going on.

And according to them they have no obligations to you:

So they post a bounty, you work for it and then they tell you that they have no obligations to pay you. I guess it's time to send those negative trust and complaints about these people to this sites admin/moderator.