It's probably OK.
This is common in lots of appliances that have 2-core mains leads.
I expect that the USB power supply inside the socket is a 2-wire design, with only a L and N feeding it.
It probably has a mains filter, which leaks by design to the PSU ground, which is not connected to the mains ground.
Thus the PSU ground floats at 1/2 mains voltage, but at a very high impedance ( ie low current ).
This is perfectly common in things like TVs etc.
I can't thing of any good reason the PSU ground is not connected to the mains ground, since it is right there and does not require any extra cabling, but there may indeed be a reason.
The first person to suggest 'hum loops' in this scenario gets strangled with a Russ Andrews mains lead.