Overall a symptom of the mess that is the VM pricing structure.
I did forget something though, those people who are presently paying over the odds would pay less if they were to simply change the pricing on the tiers rather than adding discount codes, so it certainly pays Virgin to grandfather legacy pricing and add discounts to drop pricing to current levels rather than drop the prices of the tiers themselves
You were dead right though Ben, nothing to do with being lazy but more to do with keeping those who are paying legacy rates paying those rates while advertising lower prices for new customers and older customers who persue the matter
EDIT: I'm not saying this is somehow wrong, it's what every company does.