Surely they charge the keyboard and mouse. Wouldn't at least one of them be charged via a USB cable? Plugging that to the computer would turn the peripheral into a wired USB one.
Plenty of cheap peripherals still run on disposable batteries.
I haven't been super happy with the rechargable AAAs I've used, so I might prefer specific batteries for smaller items. But most video game controllers or presentation remotes or wireless keyboard and mouse have plenty of room for one or two AAs