> Why should Apple care if I want to leave the device plugged in all the time? How does this choice remotely affect them?
Because the wireless-ness of the mouse (while also being a macOS-compatible multi-touch surface) was the selling point / feature / Unique Selling Proposition of this mouse vs. other mice (and vs. the previous Apple Mighty Mouse.)
I don't know if you've ever had the opportunity to see many "normal" people's home-office desks, but I have — I worked as a call-out computer repair tech as a teen. And it taught me something: a lot of people have a really small or cluttered "mousing area" — often arranged in such a way that, for a wired mouse, the mouse's wire gets in the way of the mousing surface.
Picture, for example, an old 18"-deep sewing desk up against a wall, on which the user has placed their laptop [effectively permanently, as its battery is long dry]; with a bunch of other things like tiny little speakers and an inkjet printer competing for space on that tiny desk, such that there is only a 8"x8" square of free space to the right of a laptop. The user's mouse is then plugged into a USB-A port of the laptop that's also on the right [mouse cable is too short to plug it in on the left!], with the port being at about the center of the laptop's side. This mouse cable now "wants" to lay directly into the center of that clear 8"x8" square of space; and even if you bend it harshly, there's at least two inches of USB-A plug + cable strain-relief that will still be poking you in the hand.
(Why do they use a mouse at all, if they have a laptop, which presumably has a trackpad? Because trackpads on laptops — especially smaller/older/cheaper ones — can be ridiculously awful [tiny, laggy, insensitive, jumpy, etc], such that this cramped mousing experience is still better than the alternative.)
In such setups, "erasing" the mouse's tether to the computer is not just for aesthetics; it's a genuine ergonomic improvement that makes it "feel" better to use the computer.
And that means that any average cramped-desk person who buys one of these new-fangled wireless mice (or a computer that comes with one) — and actually does use it un-tethered — is going to become not only an advocate for wireless mice, but also likely an advocate of whatever brand of the mouse/computer was, due to the novelty-capture halo effect. (I.e. the "if you only date awful people, you'll become obsessive about the first romantic partner to be decent to you" effect. Decency [or wirelessness] isn't unique; but if you only know it from one place...)
That viral halo-effect-induced word-of-mouth brand-advocacy created by being at the vanguard of the Bluetooth wireless peripheral transition, is the potential upside that Apple saw when creating the Magic Mouse.
And it wouldn't be one they could capture, if they allowed sheer incuriosity to lead that average cramped-desk user to never even try the mouse without the charging cable attached (or, worse yet, if the Macs that shipped with Magic Mice were set up by people who didn't even know the mouse was supposed to be wireless — thinking instead that the mouse was just a wired mouse with a "modular" cable!)
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Now, admittedly, Apple had many other ways they could have achieved the same goals.
For example, they could have just detected that you're using a Magic Mouse with one of their computers for the first time, and forced you through a little software tutorial that gets you to unplug it — and use it unplugged — for a bit.
I'm guessing they didn't go with that solution for several reasons:
• it goes against the marketing of Macs as being "ready to use for productivity out-of-the-box". Forcing you through a hand-holding tutorial isn't very "ready." (And mark my word, if there was a skip button, even the people most in need of that tutorial — especially those people — would skip it. People don't read manuals on frickin' home CPAP machines, and then die; you think they're reading that?)
• Apple loves thinking of themselves as a design company first and foremost. (Apple products are all stamped "designed in California" — that's what Apple does there, they design things.) And if you know anything about "design" as an academic discipline, you know it's all about figuring out how to shape products or information in ways that cause people to subconsciously/intuitively make certain choices. The core of Information Design is visual hierarchy — "organizing and formatting text to ensure someone glancing at a poster gets the most critical information before glancing away." The core of Industrial Design is the concept of affordances — "putting push-plates on the push side of a door and pull-bars on the pull side." Apple doesn't want to stop you and tell you how to use their stuff; Apple thinks they are clever enough to design their products such that they afford being used in exactly the intended way. And when the product's design "fights back" from having a positive affordance to idiomatic usage... they just design more forcibly, actively de-affordancing non-idiomatic usage.
• A tutorial that pops up on Macs doesn't help someone who wandered into an Apple Store; bought a Magic Mouse (a perfect "this store is too expensive for me, but I want to buy something" purchase in an Apple Store ca. 2009); went home, and promptly plugged it into... their Windows PC. Yes, people really do sometimes buy Mac peripherals and expect them to upgrade their Windows-using experience, not realizing that Windows doesn't have the particular set of multitouch gestures mentioned on the back of the box (especially not back in 2009.) The "hardware tutorial", meanwhile, is platform-neutral.