Both AB and Boeing used to allow upper surface frost on various products. However, it's in direct violation of Part 121 which states that alleviation may be provided only for the undersurface. This was pointed out to them.
So AB, being good corporate citizens, withdrew the upper surface frost alleviation in their books.
And duly got flak from all kinds of airlines because "Boeing still allows it". I've seen at least one rather annoyed presentation from AB on the topic basically bemoaning FAA's failure to enforce its own rules on Boeing.
AB was (a couple of years ago) endeavouring to do various studies to permit them to reintroduce the upper surface frost allowance; I don't know if they have completed that work yet, or if it would be enough to get around the rather black and white wording of 121.629.
14CFR121.629 (b) No person may take off an aircraft when frost, ice, or snow is adhering to the wings, control surfaces, propellers, engine inlets, or other critical surfaces of the aircraft or when the takeoff would not be in compliance with paragraph (c) of this section. Takeoffs with frost under the wing in the area of the fuel tanks may be authorized by the Administrator.