Selectively paying out people isn't considered scam for you?
Not exactly. The same reason why banks start freezing accounts of some and cheat some but not all and are still operating legally. Bounty campaigns here have done the same wherein they paid some and not others giving excuses. I would call these banks and campaigns fraudulent but others won't as they did not behave the same way with them.
Every exchange has the same history and I myself withdrew my BTC earnings via yobit by selling my coins and I know many users did not for God knows what reasons. I wouldn't consider using them at times but since they paid me, I might take a risk with few shitcoins which are worthless otherwise.
What I can see it happening is they putting something like X BTC every week/month which can easily run out (due to the amount of participants flooding the forum with 15~20 posts day all that suddenly) and some people will get paid, while others won't. I believe this happened in their old campaign (older than their first appear here this year).
Till Yahoo is managing the campaign, I feel everyone who deserves being paid will be paid. I don't see such a situation possible till Yahoo exists. Else, earlier we had nobody and hence yobit took advantage.