As everyone here has really already posted, DISH Network does not want its installers, nor any of the contractors to attempt to climb on a roof when there is a slip or fall type hazard. Snow or rain, a technician should not be on the roof. I believe even without precipitation there is something written about lightening storms too. From my end, the company says that techs are not be scheduled during bad weather, as weather may be a cause of the issue and it puts my technicians in harms way. I personally look at the forecast and see if I can find a day of thaw or evaporation in the future to prevent any hazard to my tech. On occasion where winter is long and the snow won't be clear for a long while, I have created work orders to "abandon" the existing dish on a roof (with customer permission) and install a new one at ground level or somewhere where roof access or fall hazards won't be experienced. Having "been there" I want my tech to be able to fix your issue and get home to his or her family safely.
I know when I was in the field, OSHA could not only fine the company a serious amount for violations of safety guidelines (which I think even extends to vehicle daily maintenance logs) but could actually fine the technician themselves. That personal fine was hefty (I have heard threats that it was $10k for the first incident, but that may be an urban tech legend). You can find OSHA guidelines at OSHA.gov I think.
To the OP: Yes an installer can go on a roof in winter, providing the roof is clear of snow or moisture that could potentially (not to be confused with definitively) create a fall hazard. If a tech feels that the roof is not a safe place, whether it is due to weather, moisture, or construction they can refuse to get on the roof. In order for them to be on the roof, it is necessary for the tech to be able to install the proper safety gear (fall harness) and must have access to the roof in excess of the immediate area of where the dish is or will be mounted in order to install that gear.