Well I don't think a thread temporarily being on the first page is going to hurt anything. If a service is an obvious scam, people warning about the service is going to make their potential victims have an easier time seeing the warnings. Similarly, if a service is not a scam per se, but is having problems, having a thread on the first page might alert potential customers of the problems if people are warning about the problems.
I would say that a single reply is simply dwarfed by the impact of front-page placement and by the (assumed) diminishing relevance of subsequent posts by prospective readers.
In the case of users that read in chronological or reverse order, padded replies can create enough noise to dissuade further reading.
Besides, is it really a case of "so bad we quarantine the scams" a la Investor-based games?