It's most likely so the techs don't have to carry large trash around in their trucks all day, nor be tasked with the disposal of such. They have no use for it, so they don't take it.
It's possible that it's for some legal reason(s). One possibility being that, when the service is terminated, or a something like the dish is replaced, the old dish becomes the property of the homeowner. The only things that remain the property of HughesNet, at that point, AFAIK, is the modem, power pack for the modem, and the radio from the dish. Everything else becomes the homeowner's. That is, of course, unless the equipment was purchased.
This is actually very common practice. No satellite providers, whether internet or TV, take old dishes with them. Even when a homeowner chooses to pay for a professional de-installation (instead of removing the radio, modem and power supply themselves), neither the dish, nor the mounting, is taken by the tech.