Yeah, this is why the fediverse is terrible, too. Your site admin shouldn't be the one deciding who you can DM, or which people can follow your feed.
Imagine if email worked that way.
Moderation is great, the trouble is when a single moderation standard is used across a monopolized market.
EDIT: also to note, email does kind of work this way with administrator applied spam lists. I would not want to use an email service that couldn't or wouldn't filter spam.
On my mailserver, I make the rules. I decide how my spamfilters, greylisting and blacklisting is tuned. I decide who is allowed to have an account. And therefore, I decide what is allowed to be sent and received.
As long as it's optional - and in the fediverse design it is - then it doesn't seem entirely unreasonable. Whether or not those instances actually survive and have any users or just immediately collapse into internal bickering is beyond the strict scope.
Am I unusual? When there is hate speech directed at me, I don't really care if I see it or not.
But I do want to prevent other people from seeing that hate speech (I don't want the hate to grow).
That's a necessary affordance, given the technology. A mail server could indeed refuse to forward some of your emails, and this could even make some sense e.g. as part of spam prevention policy.
It is perfectly reasonable for a user to want to DM someone on an instance that has different moderation policies from their own, and it is equally reasonable to want to receive replies to those DMs.
If the specifications or implementations don't allow that, then I suppose it has to be justified by saying that the DMs could be used for sharing copyright infringing material (or worse), and admins don't want to run the legal risk of hosting that on their servers. Legally, though, that doesn't seem any different to operating a mail server, which don't typically have Content ID matching systems on them. Perhaps the implementation of end-to-end encrypted DMs would assuage some of these concerns a little.
I think there is actually a lot of room for experimentation around how we interact online, and moderation is one of the most important areas. We need to think harder, rather than retreating to one of the two default positions.