Just a little thing called logic. I'm not saying they are required by law, of course. That's stupid. But, logic and common sense says they should. The 510 cost them no more than the 501/508 to develop, and offers no additional functionality. (hdd space is nmot a function.) And, the 510 actually provides less functionality than the 721. The argument against Dish DVR fees per receiver, especially for the 510, is clear and strong.