It is to cull high-volume bot programs, which are managed through the API. Note that every account will require a log-in to count as active, not just be posting

It is to cull high-volume bot programs, which are managed through the API. Note that every account will require a log-in to count as active, not just be posting. If a single bot account was created by a person for fun, they can easily log into it once to prove activity. But if a person (or, say, a national intelligence service) has created 20,000 accounts programmatically, they’re going to have a much harder time manually logging into every one of them to preserve them.