May I ask the reason why you think that the static electricity will be confined to VCC_USBn line?
From my understanding, it is quite difficult to predict that any specific USB pin will carry static charges.
Lets considering male type USB connector, they generally have a outer metal frame which connects to the body of the USB connector on the carrier board. The body of the USB connector on the carrier board is connected to the SHIELD signal to provide a safe discharge path for the the static charges.
The power and signal pins (VCC, USB_P, USB_N and GND) are not easily accessible, unless and until someone wants to intentionally access it.
So the chance that the static charge will remain in the outer body is higher than being at the inner pins.
ESD protection diodes RCLAMP0504 are connected to USB_P/N signal because we want to protect the processor pins from such discharge as they are not tolerant to ESD.
Let me know, if you don’t agree with my arguments. This discussion will also help me get better understanding on this topic.