Manoeuvre speed is not "set" by the stick shaker or the Buffet alert. It is a variable condition and flying at less than a certain speed for given weight and conditions will either activate the stick shaker or trigger the Buffett alert.
From the Boeing 737 FCTM: Quote:
"High Altitude Recovery. At higher altitudes above 20,000 feet, the airplane becomes increasingly thrust limited. If an approach to stall indication is experienced, nose down elevator and stabiliser trim is required to initiate a descent. This is because when the airplane is thrust limited, altitude needs to be traded for airspeed. Therefore a recovery at high altitude results in a greater altitude loss than a recovery at low altitudes".
In practical terms, at high altitude very heavy buffet will be experienced before a stall condition and recovery should be initiated before the activation of the stick shaker . At low altitudes the stick shaker often activates, there are bulletins from Boeing on this matter regarding VNAV and nuisance warnings with selection of anti ice etc etc, and recovery is much more straight forward.
Perhaps you need to look at the theory of energy management at high altitudes and low altitudes, it may help you understand the logic behind Boeings thinking.
Ref:
Stick shaker. An artificial stall warning system is required for airplane certification if the natural prestall buffet characteristics of the airplane are insufficient to warn the flight crew of an impending stall. This warning must be in a form other than visual to be effective, even if the flight crew is not looking at the instrument panel. Beginning with early commercial jetliners, standard practice has been to equip these airplanes with a stick shaker as a means of stall warning. Some airplanes also have employed stick nudgers or stick pushers to improve stall avoidance and stall characteristics. All these indications have been driven by an AOA threshold, which is usually a function of flap configuration, landing gear configuration, or both.
Because of the effect of Mach number on stall AOA, the stall warning AOA typically was set at a conservative level to accommodate gross weight and altitude variations expected in the terminal area.
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The early stall warning system thresholds were not set to be effective at cruise altitudes and speeds because they did not correct for Mach number (fig. 10). This kept the system simple. The stick shaker was set at an AOA effective for low altitudes but at too high a value for cruise. Natural stall buffet was found to give satisfactory warning at higher Mach numbers.
Later stall warning systems used Mach number from the pitot or static air data system to adjust the stall warning AOA threshold down as Mach number increased. This provided the flight crew with a stall warning related to the actual available performance. However, it also made the stall warning system dependent on good pitot and static data, a factor that will be considered in the next section on the dedicated AOA indicator.
It should be noted from figure 10 that the stall warning schedule does not follow the buffet boundary at very high Mach numbers. The buffet here is caused by Mach buffet, or too high a speed. Setting the stall warning system to activate at this point may lead the flight crew to believe the airplane is near stall and increase, rather than decrease, speed.