Why the cable screen shall not be grounded at instrument field device?
Cable screen protects the signal carried on the conductor from external interference. Cable screen blocks the external interference and noise, then directs it to the ground.
Screen shall only be grounded at one source. The grounding is usually made in equipment panel or cabinet while field device point is left ungrounded and insulated. This is to eliminate the ground-loop current that may arise if screen is grounded at both end. In this case, the ground loop current becomes a noise for the signal conductor itself.
The following wiring diagram shows ungrounded instrument cable screen at field device.