Originally Posted by marcop

The sites' owners don't care about the models because they aren't the major source of revenue for them--the bulk of their revenue comes from amateur photographers who join because they want to shoot naked girls.

The amateurs mostly want to do TFP (time for prints) or the new thing, TFCD (time for CD) instead of paying the models, so the sites' mods keep out porn shooters as they pay real money and would be overwhelming competition for the amateurs.