My understanding is that a CR voucher is a commercial voucher.

To try and understand this a bit more, redemptions and commercial ticketings are completely different from each other, with commercial ticketing obviously based on cash. The online eVouchers are actually tickets, which just hold a cash value and can then be plugged into another cash ticket to pay off the cost of travel - it's a simple piece of maths. Redemption ticketing has a different structure based on Avios and so just on a technical level you just can't add a cash voucher to a redemption, it just won't compute. The benefit to BA of issuing a commercial online voucher is that they won't lose that cash, it's easy to administer using a system that's been around for many years, and can be used without tying down staff resources. Customers can book commercial tickets with the evouchers online very easily. As soon as a customer says "ah, actually I really want to use this on a redemption" then the whole thing falls over.