Of course the ACR comes with Slotted rotors, and there is a price premium for that over the factory blanks so cost becomes an issue when you are talking about putting them on ALL Vipers.
Also, while there are several owners that buy cross-drilled rotors, some Viper buyers/owners bought it for looks, and just drive around and will never have a problem. Others however, will beat the crap out of it around a track, and drilled rotors have a history of cracking around the holes after intense braking. Doesn't seem like something Dodge would want to warranty all the time IMO.