While digital piracy is certainly a problem, the answer isn't really that nefarious. It's simply a billing issue.
You're paying month-to-month for the service. The only way they can do that is if they have a way to turn it off if you don't pay your bill. They do that by storing an "authorized until" date in your unit. Every month when you pay your bill they send a new date to your unit, about 30 days in the future. They also periodically send all the dates to all units during the month on a rotational basis, usually at night, but that might be a week before you get it.
If your unit is not plugged in and connected to a dish, it doesn't receive the new "authorized until" date, so it thinks you haven't paid your bill. When you ask them to reauthorize, they simply resend that new date through the satellite to your unit.
I don't know about Bell, but dish and DirecTV allow you to log into their website, or use an app to reauthorize all receivers. I haven't done it for years, but DirecTV used to only take a minute to get the new date.
The bottom line is the unit has to be plugged in and connected to a dish to get the new authorization date. That's why it doesn't work after a few weeks.