The rationale for this is that it is better to have twenty active citizens than four hundred inactive ones. There may or may not also be questions attached to the Culling letter, such as a census or a poll. The mailing list is the main forum of communication between citizens. If you're a citizen, it's a good idea. If not, you'll be expected to keep track of the news page to see if you're missing anything.
People are always free to leave the list.
Maybe you're getting too much e-mail or maybe you just don't want to be on it. You won't be forced to stay longer than you want to. Unsubscribe instructions are in each letter. All citizens are free to talk or discuss on the list. That's what it's for.