It's pretty terrible UX, but I can see how they came to the idea. If the mouse lasts all day, then surely the user will just charge it when they are done. However, this is a hard thing to train people to do. For people with a desk that step away at the end of the day, such as leaving the office) it is a bit easier to build the habit. However, if you are a user who uses the computer on and off all day, or doesn't have a permanent desk, it might be hard to remember to flip the mouse over and plug it in at the end of the night. Particularly if it's in your backpack. I don't personally use a mouse with a laptop, but I know plenty of people who do.

The mouse also just looks weird when flipped over to charge, and you can't really try to even charge it while it's in a backpack if you wanted to, due to the 90deg angle it makes when plugged in.

Friend told me his wife leaves it charging when she's done using it at night. But I agree it's kind of terrible UX. On the other hand... The Marathon Mouse I bought years ago only needed new batteries 3 years after me having owned / used it daily, and it came with it's own batteries.

The charge lasts months.

Not that I necessarily agree, but I've heard that Apple intentionally designed it that way because they didn't want users to plug the cord in all the time and effectively make it a wired mouse. They wanted users to actually use the mouse wirelessly as intended.

That certainly seems like a more logical explanation than literally no one at Apple anticipating such an obvious design consequence for a major accessory.

QA? QA is for companies that make mistakes. /s