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.