I agree that it's kind of a questionable decision but my understanding of it is, people can normally post public replies to tweets- but if the user blocks you, you can't post your public reply. OK (and desirable) for most people, not ok for the government to do.

There's not necessarily a perfect offline equivalent but it would sort of be like if the president tried to forbid a newspaper from running an editorial next to the text of a presidential speech- obviously unconstitutional. (not a perfect comparison because twitter users who are blocked can and do get around it by screenshotting or just linking to the tweet even though they can't reply to it)

I think the more interesting part of this is the ruling that the president's "personal" twitter account is an official government instrument.