Why do I get an overspeed warning at 250kts below 10,000ft in Microsoft Flight Simulator?
Most likely Cessna introduced a speed switch that is controlled by an aneroid.  Anytime you are below 10,000 FT MSL, the aneroid activates that switch.  If you go above 250 knots, the overspeed warning will sound.
As far as why that is the case, in the United States you are limited to a speed of 250 KIAS below 10,000 FT.  I would imagine this is also the case for most other countries.  The FAA realizes that most small airplanes with a much slower airspeed travel only below 10,000 FT MSL so the FAA is providing a safety margin to prevent in-flight collisions.
The structural limit of the airplane would not be a factor at 250 KIAS hence the red line at 300 KIAS.  I would imagine 300 KIAS deals more with structural stability than anything else.
My question... can a slow-tation go that fast?  :-)