You absolutely do assume that that network is hostile/compromised. There are a number of other networks in other places where more important communications are done. It's a giant PITA, and a lot of stuff leaks from Congress because most people believe that they are too important to have to keep things secure.
Being in Congress is not an automatic ticket to having all the country's secrets on your desktop, and things actually are compartmented quite heavily when necessary.
> Being in Congress is not an automatic ticket to having all the country's secrets on your desktop
I'm not suggesting that they have _all_ the country's secrets on their desktops, but surely the speaker of the house needs access to their emails on their desktop. Those are private, but without them, why even give the speaker a desktop at all?
Being in Congress is not an automatic ticket to having all the country's secrets on your desktop, and things actually are compartmented quite heavily when necessary.