The reason is sneak circuits - a circuit unexpected in the design, perhaps from degradation of the hardware. Sneak circuits can and do put line voltage on the D

The reason is sneak circuits - a circuit unexpected in the design, perhaps from degradation of the hardware. Sneak circuits can and do put line voltage on the DC output of bad chargers. There is an EC standard 60601 for medical equipment and there are power supplies that conform to it. They have to be well protected from sneak circuits and they are expensive. Realizing both inexpensive and safe is best done by not allowing the device to run when it is connected to line voltage.