The reason is the prohibition under Sharia law of depictions of the human form. Somehow covering the face excuses the fact that the rest of the mannequin is the human form, but Allah is apparently satisfied by a cloth over the face according to ISIS.
Mannequins continue to be the focus of clerics who seek to block their corrosive impact on the morals of society (here and here).
On the positive side, this is the most moderate move of ISIS which notably did not behead the mannequins in a public execution after forcing them to convert to Islam.
