The recommended technique for inspecting beehives for AFB involves shaking the bees off each brood frame before searching the frame for abnormalities in the brood. Obviously this technique takes more time than simply removing frames and not shaking off the bees. Leaving bees on frames tends to be part of inspection routines where only a few frames are inspected, since each frame is pulled from the centre of the brood nest and put back in place before another frame is removed.
Bees obscure visual signs of AFB
The chances of finding light infections of AFB (less than 50 infected cells per hive) are greatly reduced when there are so many bees on the brood frame that a majority of the cells are obscured.