These rooms have ventilation (and also usually a heater, cooling, dry mode). The ventilation can be set to 24/7 mode, which runs at a low setting, always on. Japanese houses/apartments also typically have a house ventilation system, meant to run when it's humid. Mold in general is a problem here, so ventilation is incorporated well.

It's fun in the winter when you're running humidifiers but forget to turn off your house ventilation and don't understand why you're still so dry.