Hi nitpicker330,
The BUSS doesn't know anything about IAS either
The Stall warning threshold Alpha changes with Mach Number.
The BUSS green area (safe Alpha) cannot be the same at sea level and FL 350. Therefore the recommended MAX FL for BUSS use is FL250.
See page 89 of
http://www.bea.aero/docspa/2009/f-cp...p090601.en.pdf
"The disabling of this warning was probably due to the fact that, between 13.4 and 15.5 and then between 17 and 19, and possibly at other times, the three Mach values were abnormally low (three Pitot probes iced up). The warning triggering threshold then suddenly increased to values of about 10°, much greater than the recorded angles of attack, which led to the warning stopping."
Edit.
See Page 7 of
http://www.ukfsc.co.uk/files/Safety%...ary%202011.pdf
"For a given configuration and at a given Mach number, a wing stalls at a given Angle of Attack (AoA) called AoA STALL. When the Mach number increases, the value of the AoA STALL decreases."