It was explained over at DAZ many years ago, probably before this thread was even made.

It's part their location (Utah), but it's mostly to do with credit card payment systems, CC companies charge a 2.5% surcharge on every transaction, some sites will swallow that charge, while others pad the bill to cover it, but that is for "PG13" classed sites. With more adult orientated sites (non sexual) that surcharge gets increased by quite a bit, and with sexual sites some CC companies will refuse the transaction, which is why most of them use "middle man" companies like CCBill for payments.

So DAZ decided to stay PG13 to avoid all of that.