According to Airbus, using reverse below 25kts causes snow to recirculate into the engine inlet and potentially cause a flame-out. Not noticed this myself and have selected idle reverse on a snow-covered taxiway when wheel brakes were doing absolutely nothing to control the speed. Weirdly there's nothing in the FCOM prohibiting reverse use on a taxiway, although it may be that it falls into the same warning category of "do not land the aircraft upside down" or "make sure that both wings are firmly attached before flying".
Edit: you might be misinterpreting the FCOM. I think in this case Airbus wants the reverse stowed at slightly higher than normal speeds.