§ 121.629 Operation in icing conditions.
(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.
The crucial part of the reg (that's FAA, but it was easiest to find) is bolded - "other critical surfaces". Someone may have defined the A321 fuselage as "critical", in which case it must be deiced. Certainly it's been defined as critical for many rear-engined aircraft, which consequently do require fuselage deicing (I think it was an engine ice ingestion concern originally)