Even he did pay half of all participants, it's still fall to scam if he didn't want to pay the participants when they're eligible.
It is still their campaign and they have the right to decide who is eligible and who is not. So neither you nor I can say who is eligible.
The campaign is confused and badly set up, it is to be expected that it will cause a lot of controversy. It's simple, if you're not ready to take risks with this kind of campaign, if you don't like the management, don't apply.
I think you're oversimplifying it here. If the rules are not clear and the participants did the work, then they are scamming users by just kicking them out without an explanation.
It is their right to have rules and not pay based on those rules, but if the rules are not posted how can they expect users to comply with them? There's interpreting a rule incorrectly and there's breaking a rule but in this case looks like there were no rules to comply with.