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.
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.
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.