Whether admit it or not, sometimes I suspect they might be paid to give a good review of one of the casinos. This happens because we don't know who is judging and their review site doesn't have clear names of judges. Plus, I also don't know if they actually try to play at the casinos they review or just rewrite what the casino says.
Regarding that specific issue concerning about being paid or not, I doubt that most good reviews are paid. The promoted site itself might not even recognize or know some of the game review sites. Generally speaking, most review sites need to have a good set of reviews shown on their site as they are also marketing their affiliate link. Referrals are required to have a real deposit on that site before the commission will be received by the promoter.
If these review sites lure their visitors to a bad site, then they are just wasting time on managing their review sites. What's the sense?