How does this risk go away in the final 2-300' of an approach?
Its simply a measure of time. Compare how long it takes to fly the approach from reducing power to being at 200-300 feet compared to the time from 200-300 feet to landing. The reason for selecting it off at that point is to ensure that you have full power for the go around.
In aircraft such as the PA28 you cannot operate the carb heat at the same time as applying power wheras in a Cessna type aircraft you can easily operate both controls at the same time. By turning the carb heat off at 200-300 feet there is little risk of ice forming in that short period of time and less risk of trying to fly away with less than full power. It also reduces the workload on a goaround.