I can conjecture some reasons. Datamining would be high on the list, since MS can read your mail and thus market to you more effectively. Governments might also be happier if MS can provide them with one-stop access (like Prism) to otherwise self-hosted e-mail accounts proxied through a central server.
BUT, more likely, it's some sort of "check in" or "dial home" feature that isn't working. Or plausibly, maybe they count "local user checked email" and their analytics endpoint is down.
Regardless, it's definitely surprising that a traditional desktop application would be blocking when only polling from local/trusted data sources.
I'm being an ass in saying this, because I'm aware human motivation is a beautiful kaleidoscope of variables, but I would have guessed at the venn diagram of 'those willing to compromise and use a Microsoft product' is mutually exclusive with that of the 'I'm hardcore enough to self-host email'.
I will say, however, that Outlook used to be one of the better desktop email programs, so I'll grant some legacy lenience.
I use Gmail's web interface for my Gmail account (which, I'll admit, makes my little rant above more than a little hypocritical)
And in my self hosted email setup I use RoundCube (web interface).
Na, that would simply be the kaleidoscope part you mentioned, wouldn't it? For what it's worth, I found the combination just as peculiar as you did.