It doesn't have an isolated ground (the focus was on size and cost), but this shouldn't be an issue. All spec-compliant USB ports contain self-resetting overcurrent protection - in practice this is almost always via a PTC resettable fuse on USB GND. For what it's worth, I haven't had a single complaint related to the grounding in 3 years.

Ok, but how would you (for example) measure the voltage difference over a current-sensing resistor that is positioned at the high-side of (say) a 48V rails?

(If ground was decoupled, that would be simply a matter of attaching probe and scope ground at both terminals of the resistor.)