All AT&T Customers are Dish Network customers anyways (Dish owns the customer) so that would not make a difference.
My only guess is:
* AT&T thinks of Homezone as an interim solution until Uverse is fully ready (good luck with that!)
* If AT&T installs the Homezone DVR along with their high-speed DSL service with their own router, it becomes pretty easy for them to swap out Homezone with Uverse later on
* Dish Network, recognizing that the Homezone DVR is both more advanced and more desirable than their own 622, doesn't want to lose any customers to AT&T's database, so they require AT&T to enforce the "new customers only" restriction as part of their co-branding deal
* AT&T agrees to the stipulation since Uverse is still rolling out (and glitchy, from many accounts) and they are desperate to offer a triple-play solution to compete with the cable companies
So the customer who wants a Homezone DVR (and who would happily pay for one) is out of luck.