Why ptc's restrict maximum number of direct referrals to some limit?

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The main point of discussion here is that why ptc's limit number of direct referrals anybody can have? As, we all know that there are just one or two trusted ptc websites and they both have imposed limits on maximum number of direct referrals. Can it be disastrous for any ptc to allow unlimited number of direct referrals. According to my understanding, any ptc's business model should be in such a way that click rates are matchable with the advertising rates. If, people can directly bring unlimited clickers to the website then it will attract new advertisers automatically. So, it should be harmful for any ptc. Can anybody present here throw some light on this topic?