If this was controlling a receptacle where the installer has no control over what the user will plug in I with agree you but with fixed loads the switch is sized to the load. The answer is in the nameplate rating of the furnace.
404.14(A) Alternating-Current General-Use Snap Switch. A form of
general-use snap switch suitable only for use on ac circuits for
controlling the following:
(1) Resistive and inductive loads not exceeding the ampere
rating of the switch at the voltage applied
(2) Tungsten-filament lamp loads not exceeding the ampere
rating of the switch at 120 volts
(3) Motor loads not exceeding 80 percent of the ampere
rating of the switch at its rated voltage
That is spelled out in 404.14(F)
(F) Cord- and Plug-Connected Loads. Where a snap switch or
control device is used to control cord- and plug-connected
equipment on a general-purpose branch circuit, each snap
switch or control device controlling receptacle outlets or cord
connectors that are supplied by permanently connected cord
pendants shall be rated at not less than the rating of the maximum
permitted ampere rating or setting of the overcurrent
device protecting the receptacles or cord connectors, as provided
in 210.21(B).
