Earlier quoted context omitted.
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…
I've noticed there are a lot of rude replies under most of Trumps tweets. I'd think it would be normal to block that kind of thing but I guess it's an odd situation - Twitter wasn't really designed for the president of the US. For example - just the most recent https://twitter.com/ronrobiins/status/999283009957318656 - presidential ain't what it used to be.
also this is not the first president to whom people have said rude things. If the definition of "presidential" has changed it's not because of people calling Trump a "parasite".