This is actually a common misconception. If you have sealed ducting in front of the rad, then what you said would be true. However, at speed, it is still more difficult for air to go through the rad than around it. Without sealed ducting FORCING the air through the rad, it will still predominantly go around. However, at speed, there is sufficient airflow moving across the front and around the rad to pull heat away; airflow which is not present at low speeds/stop & go traffic, giving the results that have falsely been attributed to air moving through the rad.

I have done quite a bit of testing with this over the summer. The results have been quite staggering.