Because it isnt Visa or Mastercard that issue the facilities, its individual banks. So its not that hard to get a facility, but can be expensive (lots of capital required as security)
But eventually Visa and Mastercard find out, and then they get the shits, but once you have a merchant facility, if you are doing any sort of volume, and they take it away from you, then potentially the damages could be large. The problem with PayPal for them was that even though the volume wasnt huge (I think when they had 7 million members they were still only processing $7million a day), they were very much in the public eye. And Visa and Mastercard would have looked pretty uncompetitive had they yanked everything from under them (think potential anti-trust issues) - this is of course different to your typical adult site, which doesnt have a great public profile, does even less processing, and no one in the community gives two shits about, so Visa and Mastercard can do what they like.
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