The way I understand it is that you are paying up front for the upgrade, whether you do it through DirectTV or go to best buy and purchase it. They seem to rationalize it in comparison to a car lease....you pay up front, plus still have a lease fee.
Look at it this way, all leased receivrs have a $5/month lease charge, from the lowest SD model to the HD DVR. The upfront price that you pay when you get the receiver should be looked at as an upgrade fee, you get a basic receiver for free, want a SD DVR or HD receiver pay a $99 upgrade fee, want a HD DVR pay a $199 upgrade fee.
Using your car comparison you could probably get a lease charge of $400/moth for a Chevy and a Caddy, but you pay a large upfront charge on the Caddy to get the $400/month payment.