Dish and DirecTV want you to leave the dishes up so if you ever move out and new tenants move in, they'll be enticed to consider getting service, assuming it's a current dish. Or if you decide to go back to the service, they don't have to contract out the installation (they almost always have some other service install the things.) If the dish is obsolete, they don't want it back anyway.
Cablecos tend to want the current boxes back because they can always lease it to someone else. They never sell boxes in the United States, only lease. If you have one of their boxes, just take it by their office, tell them you canceled, and get a receipt. You can't use the box anyway. It's useless without Comcast sending a signal to it.
Don't believe everything on the Interwebz! A duck's quack DOES echo!