I agree with memnoch, I think (assuming I am reading what you said right). To get HD you have to pay for the box ($5+$5 for the DVR) which I assume they are making SOMETHING on. Plus, you have to go the digital packages. In my area, that costs me $12 extra. I would NOT get it if it wasn't for the HD channels. My only theory is that they make enough from the digital tier upsell to cover the HD expenses.
Personally, I'd like an HD tier, if I could get it without additional SD digital content. Give me all the HD I have now (ESPN, INHD1/2, TNT, local affiliates, and Discovery). Add in a regional HD sports channel (FoxSports, for me), and an HD movie channel of my choice (so, for example, if you picked HBO you would get the regular analog HBO signal and HBO HD but not HBO2, etc). Charge say $15 for it and I'd buy, assuming they were 'committed to adding more HD content whenever available'.