There was a similar discussion and I will quote the most merited post.
It's just like any other deal. You have a promise to pay and a risk of not getting paid. Like e.g. if you buy a trinket and risk that the seller won't deliver. Nothing wrong with positive trust here, if it's not extorted/solicited/begged/reciprocated etc.
Edit: another way to look at it - if the opposite happened, i.e. campaign manager didn't pay on time/correctly/etc, would they get a type 2/3 flag (contract violation). If so, then positive trust rating for paying on time/correctly/etc is justified IMO.
This mean most people agree if someone left positive feedback even it's "free money" by your own definition, because when people are participating in contest, they also carry the same risk that Bc.game will not give the reward. You know, it's possible to run a contest and the sponsor can accuse or doing anything in order to not give the reward.