The light gain will pay for itself in more sales.
Also pay for itself again by not having to return the device for firmware update, instead the owner connecting it to Ethernet themselves.
There is opto couplers to limit electrical surge on Ethernet from damaging the rest of device, and most likely that surge will destroy more items, not just the router. I have lightning destroy some electrical devices in my house (and start fire), but my Ethernet network was not impacted.