MarkT:
The Uno and Mega have power-switching circuits that detect Vin
and switch over to USB only if Vin is low.
That's the case we're talking about.
When Vin is low the 5V from USB goes directly to the output of the 5V regulator.
The regulator has no input, the output is connected to 5V. Every Arduino in the world that's running on USB power is doing that as we speak.