You missed the point
the HD and the SD dupplicates are part of the basic package on VOOM. On D* and cable they are part of a higher tier package. D* & Cable can't drop the SD channels because not everyone can receive the HD channel.
Since everyone with VOOM can view the HD channels there is no reason to have the SD channels. Once VOOM has several million subscribers they will have the weight of the subs behind them to demand the SDs be removed. I think the service will peak at about 3 million afew years from now. They don't have that now and can't make that demand on the channels
I wouldn't doubt that if D* or Cable put ESPN-HD in the basic package and all subs could view it the SD version would go away very quickly because they have the subscriber base to threaten removal of ESPN all together. But since they charge extra for it they aren't going to ask that the SD be removed.