That's why a specific current pulled over the supply pair would be more useful with a fixed 5V supply. The supply ground is often a different gauge in any case to the +, so the loop should be tested.
In practice, it just doesn't matter. I use some 3-5M USB 2.0 (A-B) for audio and test devices and have never had an issue with either current draw or low voltage issues. Come to think of it, I don't ever remember windows shutting down a USB port for overcurrent, ever. Even back in the P4 laptop days running XP.