Where's you sense of humor? The cut out allows clearance for the thumb when you use a stripper clip to push down 5 cartridges into the magazine. I thought everyone knew that. Remember that the guns were made in Germany and that in the winter soldiers and hunters wear gloves. You can hold the cartridges down with your left thumb as you close the bolt. This allows to keep an empty chamber or to have a round in the chamber and increase capacity to 6 rds. One maker of new super expensive CNC machined 98 actions charges $300 to incorporate this feature in their action.
One drawback is that this weakens the left wall of the action so try not to throw one hard against a concrete wall and you'll be OK.