To the mix of questions and comments of USB and Ethernet, such as why is Ethernet galvanically isolated, and USB is not:
Read up on USB history, and its mandates. It was to be a "low cost", "short distance" (5 meters) signaling port fit for computers at home and business, and low cost above all other needs. USB also had the mandate to improve data rate performance over parallel-printer and RS232 ports.
USB was to be put on all PC's manufactured. Whether the user needed it or not. That means it must be low cost. Parallel-printer ports and RS232 ports and the large associated connectors, had a serious cost penalty on all popular computers. And that made PC's and laptops, costlier, bigger, heavier and more power consumptive. USB, thus to be very "low cost", has no transformers to accomplish galvanic isolation. And it makes it easy to provide DC power to the peripheral. USB data signaling, for lack of a better phrase is "semi-differential". That is the current in the + and - lines of the cable, are about 95% numerically opposite matched (the + and -, are always slightly in error, of not being a perfect opposite current value), as a different transistor set drives each net, + and -.
Ethernet's mandate was and is; "reliable", "medium distance" communication, and low cost. But reliable and medium distance come first. Medium distance of 100 meters needs galvanic isolation very much. If two devices (such as a switch and a PC) were connected across two buildings with a few volts of ground potential difference, that's a rather bad thing, and unintended, unwanted, ground current will flow in that data cable. And that unwanted ground flow, can have all sorts of bad effects of harming the data quality and harming the equipment's, to possibly even endangering people.
Ethernet, also has different transistor sets driving each + and -, however the signal transformer shorts the + and - together, and thus the final + and - current flow is nearly a perfect, opposite match, down to nearly a single electron. Thus true differential signaling is achieved. True differential signaling allows the signal voltage levels to be further reduced, and the cable distances traveled, to be increased, and for unwanted EMI to be reduced.
Later came PoE for Ethernet. PoE's mandate was to bring "low cost" DC power to peripheral devices, ie, VoIP phones, Cameras, & Door-Access units. The PoE typically power exits the common Ethernet switch, to multiple devices, up to 100 meters in opposite directions. That PoE (48 to 57) VDC is a "star" connection to all the devices. That means the multiple power using devices "PD", share a common supply (this is NOT isolated power, per RJ45 connector at the PSE). Therefore it is a GUILT-Edged "shall" that the PD's shall maintain power isolation (as per the IEEE 802.3 standard), even on the PoE power inputs, by way of either an DC-to-DC isolated converter supply in the PD, or the PD is entirely in a non-conductive case, and never are its circuits ground plane connected to the local ground of the building or other nearby equipments (such as really low end cheap peripherals). Unfortunately, IEEE 802.3 at PoE standard does not spell this out very clearly.
Summary: Ethernet has transformers at both ends. If even a transformer failure happened, galvanic isolation from PD remote device, to the PSE in the Ethernet switch is not lost.
PoE, gives up DC power isolation, (for the sake of low cost) at the Ethernet switch, and leaves this isolation "shall" up to the PD peripheral manufacturer. Nobody is really checking on these manufactured items.
If IEEE put a reward bounty on the violators, that would improve the situation.
The new PoE standard, IEEE is pondering for even higher voltages and currents, for more PoE power, should move toward some improved quality and safety. These should be on only commercial/industrial grade or better installations:
1) full power isolation at the PSE, for each connector.
2) required test reports for PSE & PD power isolation, that are filed, and downloadable for the public. To include the wiring diagram of the PI.
3) at the manufacturers cost, maintain a server, with a list of all PD's that meet the new standard.
4) consider making an industrial grade standard, if the cost of these improvements are too much for the low end consumer markets, yet still provide the serious level of standards, safety and traceability of industrial needs.