The postal service is a federal service. If email offerings were, say, a municipal service, it seems like the Feds would have a much harder time regulating it. The federal government in the US can certainly put pressure on municipalities, but it wouldn't be compulsory - for example, university emails don't need to be used as a primary email except for specific, university-related circumstances.
This argument could easily be extended to private businesses. The government has the capability of putting pressure on private entities to compel them to treat email in certain ways. The rule of law prevents such things, and I don't really want to start a thread about how the current administration has no regard for the rule of law.
Furthermore, not everywhere is the US. Why hasn't Germany done this? Or Australia? Or Singapore?
