Why is carburetor heat unnecessary at full throttle?
The icing you are talking about is called "throttle ice" and actually just forms around the edges and back side of the throttle butterfly.  It's not really related to the venturi that is upstream; it's from the pressure drop caused by the restriction of the throttle plate (the high vacuum on the back side you might say) along with the presence of vaporizing fuel, which massively boosts the temperature reduction caused by the pressure drop because the evaporating fuel is absorbing a lot of heat from the air stream.  

It's the temperature drop from this pressure drop at the butterfly, plus the effect of evaporating fuel, that turns ambient humidity into carb ice.  On a fuel injected airplane you only have the butterfly valve without fuel present, and the temperature drop from that alone isn't sufficient, and therefore you don't need carb heat on injected engines (just an alternate air source in case of impact ice on the air intake).

Anyway, when the throttle is wide open, the pressure drop downstream of the butterfly caused by the throttle restriction is minimal. As a result, the chilling effect of the pressure drop and of evaporating fuel is also minimal. Because of this you won't get ice formation as readily (not impossible, just relatively unlikely) when the passage has say only a 5 or 10% restriction at WOT, vs say, an 80% restriction at a low power setting.  For a given ambient temperature/humidity, carb ice always forms more easily the more the throttle is closed.  

One of the worst airplanes for carb ice was the 65 hp J-3 cubs where the carb receives very little conductive or radiant heat from the engine due to the carb mounting and the cowl with the exposed cylinders.  Descending with the engine at or near idle on a 65 deg F high humidity evening was begging for a carb ice related stoppage, and liberal use of carb heat was advised even if there were no signs of icing.  Carb ice at WOT wasn't too much of a problem, but you can never be sure.  Always err a little on the overuse side with carb heat, especially on Continental powered airplanes which are a bit more sensitive to all this due to the way the carb is insulated from the crankcase, at least until you get to know the engine's quirks.