My problem with them not offering it, is that Apple is telling you how to view your media. They don't do that with music - they have a CD drive and an app that plays CDs. They have a DVD drive and an app that plays DVDs.
There is no logic to suggest that the only reason they aren't supporting bluray is because of their itunes video store. Based on that logic they wouldn't play DVD/CD either. The simplest solution is likely just cost. Although offering it as BTO they could cover the licensing cost with that and not impact all users.
They sure did offer cd and dvd drives, but is it not possible they were doing so because most of their software was bring distributed via those mediums? Do you really think they will continue to cling to those now useless devices?