Is referral back commission(s) allowed for gpt sites?

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As a way to increase my earnings from PTC sites like clixsense, ayuwage, etc., I sign up from the links of members that offer referral back commission or RCB. I see their offers all the time in forums, ads, and others sources. I rarely or rather never see them for GPT sites like cashcrate, prizelive, or squishycash. Am I just looking in the wrong place? Or is it a violation of gpt sites TOS to offer RCB? Can someone please provide some info. I plan to start offering this to gain referrals(if it is acceptable).