Why ptc's restrict direct referrals to some limit?

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As we all know that there are just one or two ptc websites that can be trusted. My main point of discussion here is why do they restrict maximum number of direct referrals to some limit. Will it be disastrous for ptc if abybody can have unlimited number of direcr referrals. The business model of any ptc should be in such a way that the advertising rates should be mathable with the click rates and lot more things. If people can directly refer more clickers then automatically it can attract many advertisers. Can anybody here throw some light on this topic? Why they restrict direct referrals to some limit?