Edward, Because they said so!
I will go out on a limb here and just give a wild guess that when a cook top is designed it is done so with the knowledge that it may be tapped from a 50 amp circuit per the NEC. To that end the wiring inside the cook top is heavy enough to trip a 50 amp overcurrent protective device in the event of a short circuit or ground fault. Overload is not possible as the connected load will never increase.
I would bet that there are no conductors used in cook tops smaller than 14 AWG.
Take a look at the allowable ampacities for cords and fixture wires in 240.5(B)(1) & (2). I realize that those sections are not directly applicable but I believe they are a good indication of what size conductor the NFPA believes can safely trip a 50 amp overcurrent protective device.
Now think of the typical 120 V exhaust hood, the conductors inside are likely to be 18 AWG. Going back to 240.5(B)(1) & (2) it seems the NFPA believes 18 AWG should be on a 20 amp overcurrent protective device max.
Even though we do not think about it much, the overcurrent protection we provide is also the overcurrent protection for the utilization equipment.
IMO this is one of the reasons you can not protect a 20 amp outlet with more than a 20 amp overcurrent device or in the case of a electric water heater you can not have overcurrent in excess of 150% the nameplate current.
All of this goes hand in hand with the UL listing of the equipment.
Take all this for what it is worth, it is just my opinion.
[This message has been edited by iwire (edited 08-05-2004).]