Apple put the Magic Mouse's charging port on the bottom, again
theverge.com
theverge.com
Yes, there are many items than can't be charged and used simultaneously, but if it is an option, it is an option that should be used. Clearly for a mouse, it is possible.
Because of environmental concerns modern USB cables are fairly stiff. What would the usability of this fairly light mouse be with such a cable attached?
This design also almost guarantees the battery in this device will not be connected to a charger for long durations. That may be good for keeping the battery healthy.