I can't see any legal reason a payment processor couldn't pay out in bitcoins. If there is the same AML/KYC followed as when paying out with a bank payment, then there shouldn't be any reason bitcoins couldn't be paid out instead.
More likely what would happen is that the merchants attracted to this might be the ones who have a higher risk of seeing chargebacks. It may seem that there would be little difference between a merchant processor paying out for the charges via an ACH and then the merchant immediately sweeping those funds to a bitcoin exchange versus the merchant processor directly paying out for the charges in bitcoin. Probably a difference is that when simply tossing the proceeds over to the the bank account there is less responsibility for the merchant processor (e.g., doesn't need to register as a money transmitter maybe?)
For the same reason, that's probably why there are no methods to receive other digital currency methods like Liberty Reserve either.
One other possibility is using LiqPay as a credit card merchant processor. VirWoX and BTC-E, for instance, both use it in exchange for hard money.
And, from another thread:
You can then use those CashU funds to purchase bitcoins if you want (or withdraw to your bank.)
- http://en.bitcoin.it/wiki/UKash
- http://en.bitcoin.it/wiki/Payment_methods
- https://bitcoinnordic.com/cashu/index.php
Also LiqPay is another method and they do accept credit card. LiqPay then apparently can be used with OKPay, and then used for purchase of bitcoins.