Sometimes this is just a forgetful thing, sometimes it's laziness, often it seems to be that the repair person isn't even aware it's supposed to be removed.

Leaving the plastic on isn't so bad if the little tag is cut off, sometimes it's done to gap the display away from the chassis slightly. Unfortunately if the little tag is left attached it causes a pinch and crush problem to the edge of the delicate LCD display and frequently permanently damages it.