Probably the same reason they don't allow Super Joeys on multi-Hopper accounts. Even if someone like you or me knows how to get it going, we are a rarity, and most people would probably be a headache to support such installations.
One theory I have is that it boils down to Dish's rather outdated switching technology not being able to support the setup without multiple switches. Yes, you can have a ViP with a single Hopper on a Dish 1000 dish that supports six tuners, however if one were to have two Hoppers, they would need two switches in order to add a ViP receiver, or another dish. With the 211, they will most likely be using it separately with their tailgater, so no complications there.
Another theory is that people would be asking why they can't see the recordings between the receivers.
Then of course, there's the account stacking theory.
Last but not least, is the "because we can" theory.
I'm just glad that they allowed for me to have four Hoppers.