Why is the usb shield always tied to ground (GND pin)?
This question has been discussed over many entries on SE EE and elsewhere.

See 

USB Shield. To ground or not to ground?
How to connect USB Connector shield?
USB 3.0 Hub shield connection

Does an overvoltage TVS for USB hot-plugging connect to shield ground?

and most elaborate answer is provided here, Structure of a usb 2.0 connector [duplicate]

It explains that the shiled serves two contradictory functions, (1) provides the path for ESD discharge, and (2) shields EMI from noise of internal digital ground. So the trade-offs must be made, depending on particular shape of USB device - handheld, grounded enclosure, etc.