I just got the 510 on the upgrade plan (committed for two years-free plus paid the equipment fee so no CCA involved). I would consider it a trade-in plan since if you were replacing a older receiver with a new and improved model. I think they are using the same thinking like car dealers. If you want to get a better car, you either use your old car for trade-in value (which you really never get the true value of the car-has anyone ever got a great deal on a trade-in) or you sell your car on your own at the price that you want to get for the car and then use the profit to purchase a new car.
I believe that Dish realized that many customers would want to either keep the old receiver and later sell the equipment for a profit (which I think if you own the equipment you should have the right to do so) or just temporary use the 510 as an additional receiver and later disconnect and remove the old receiver.
Dish loses money on the equipment that it sells, especially on it free equipment promos. They make their money on the programming that we buy. The more programming we have, the more money Dish makes, so they can offset the lost of giving out free or reduced price equipment.
In my case, my old 7200 I replaced was a free receiver I got when I signed up for Dish, so even though I did seriously consider just selling it on eBay or elsewhere (heard people were getting around $200), I just decided to do the upgrade. Maybe in the future, I would sell the receiver first then just purchase the new receiver at the reduced price