HR's are 2 prong because they ground through the coax cable and not the home ground, All dishes are supposed to be grounded to ground the entire system not just the dish.
Actually, you've only covered half the reason. Since satellite receivers are
all connected to the dish via the coax ground, they have no choice but to share a ground. If they had 3 prong plugs, you could create a ground loop between two receivers. This would cause current to flow from one receiver to the other (direction depending upon which was at the lower ground potential). This is not good for the input section of the receiver. Therefore, the individual receivers are allowed to float and everything is supposed to be grounded at the coax cable premises entry point. If the dish is not grounded, then your entire satellite reception system is ungrounded. This means that if you happen to touch a metal part of the satellite system and a grounded item (say a pipe, a lamp or anything with a 3 prong plug) then YOU become the grounding point for your antenna system.
Dish Network Customer from 1998-2001
DirecTV Customer since October, 2001
Moderator, DBSDish.com 1999-2000
Co-Founder and Administrator, DBSForums.com 2000-2006
DirecTV: HR34-700 (1TB) / HR24-100 (1TB) / HR24-500 (1TB) / H
R21-700 (320GB) / HR21-100 (1TB) / 2 H25s / C41-500 / SWiM16 / Nomad / CCK
FiOS: 2 Tivo Roamio Pros (6 TB total) / 5 Tivo Minis attached via MOCA