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.

although usually while replacing the LCD & Screen .iPhone is already turned off .but it is still with power if you don't disconnect the battery plug .so if unfortunately some short circuit occurs during the process ,such as misalignment for plugs's possible to burn the LCD & Screen ,or even the logic board.

it's always a good habit to remove the battery plug to prevent unexpected damage for your iPhone .