This is coming from an American, so this explanation is going to be a bit U.S.A.-centric(may also contain inaccuracies or missing details):

The United States of America is federated under a central authority to facilitate cooperation, trade, and freedom of movement; this is why we call the central government the federal government. Canada’s provinces are similar in this respect, as are the states that make up Mexico.

Lemmy and the like are federated without a central authority, thus having no higher authority within this framework enforcing rules they must not infringe. Each member makes its own rules in its own territory, choosing whom they wish to federate or cooperate with. When moving to another member, or even non-member, like member states or territories, you need to move to a new address within the member you wish to join. Also like member states or territories, there is generally freedom of movement without having to take up a permanent residence in another. Email behaves similarly.

Perhaps it is best to think of each instance as a country or state. Technically, federation does rely on a set of rules to communicate — the protocol in this case — but it still remains true that each member is not beholden to a central authority and may choose to or not to cooperate or communicate with whomever they wish.