Can somebody explain this to me? About PTC channels.

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hello Mylotters, there are some PTC sites that give say that they are not meant for cash but advertising only. What i mean is that they say the amount that you earn by clicking is on for advertising and will not be paid as cash.They also require you to be a member of a payment processor like Paypal etc. Now suppose you have earned the reqd minimum amt then i suppose the amt will be credited into the pay-processor. If that is so then what will prevent me from encashing that amt rather then purchasing advertisements? Why should i buy advertisements when i can benefit from getting cash. They also say that if we ask for cash our account will be terminated. All this is confusing me. Can somebody explain this to me?