You can find it in our terms of service as well as the URL rules that we don't allow any PTP links. All members agree to this before even joining so I don't know why anyone would even question it. It is mainly because of two reasons:
1. The lack of control as to what is on the ads. Framebreaking code, geo targeted pop up ads, adult material (even in banner ads placed on a site that is ok in general) is not allowed.
2. It greatly reduces the quality of manual traffic when there are people surfing thousands of pages to promote a site that will earn them a fraction of a cent, instead of something that will generate real leads and sales and a recurring income.
Hungry For Hits