I believe that was intentional, to prevent people using it plugged in, which would mean most people would keep it plugged in all the time, so it wouldn't be a wireless mouse anymore, but also degrade the battery lifespan.
It was intentional to recycle the design of the Magic Mouse 1 which used AA batteries. The Magic Touchpad and Keyboard came out the exact same day as the Magic Mouse 2 and they don't share the Magic Mouse 2's stupid design. They both have perfectly usable ports on the front and even work when wired without pairing.
Maybe they should have made the batteries replaceable and make it operable without batteries installed.
Or just ship a wired version for the people who want that.
Cycling the battery continuously is worse for lifespan.
Right.... So.... Add some charging circuitry. Is it a problem if people don't use it as a wireless mouse anyway.
Yes it's very apple to force users to use devices how Apple wants, but that isn't a particularly good reason.
I also believe that was intentional. But the reason was the typical Apple / Jobs hubris of knowing better than the users. The desktop looks cleaner with fewer cables, so they wanted to enforce use without the cable plugged in.
I don't have a source for this, but I'm pretty sure I've read something like that a long time ago.