It is not really a big cost because you only really need to pay the platform fee (you control both sides of the transaction do the only real cost is the middleman).

Reviews are super important and it is hard to get booking when you have 0 reviews so it is not surprising that some hosts would spend some dollars for 3-4 fake positive reviews to kickstart their property (many people will bounce off the ad without at least a couple positive reviews, so it changes your business radically)