It's a severe discredit to the major operating system vendors that plugging in a USB stick can still compromise a system. If a USB device identifies itself as a keyboard, the system shouldn't accept its keystrokes until that keyboard has typed the user's login password (EDIT: or the user explicitly authorizes the device using a different keyboard). If it identifies itself as a storage device, the filesystem driver sh…
Because very many people (and more importantly, businesses) have obscure buggy printers from the 90s or the equivalent thereof.
If you plug in an old printer, you know you just plugged in an old printer; you can load the old-printer device driver and it probably won't exploit it. But if you plug in a USB stick you found in the parking lot, and it asks you whether you just plugged in an old printer, then the game is up; you know it's a tricky device, pretending to be something it's not in order to target a security vulnerability.